[Feat] (MG_Impl/GLImpl): implement glGetFragDataIndex

Fill the stubbed GL 3.3 Core glGetFragDataIndex, mirroring its already-
implemented sibling glGetFragDataLocation: validate the program object and
link status, then return the fragment color index the name binds to.

Every active user-defined output uses color index 0. MobileGL does not yet
track dual-source (index 1) bindings -- glBindFragDataLocationIndexed and
the layout(index = 1) qualifier are unsupported -- so the result is exact
for any program that does not use dual-source blending; a name that is not
an active output (including gl_ built-ins) returns -1.

Tests: assertions on the existing linked-program test (valid output -> 0,
unknown name -> -1) plus a standalone invalid-handle case. The invalid-
handle test drains the error queue it produces so no stale error leaks
into a later test (the ProgramTest fixture does not reset it). ProgramTest
24/24.

MobileGL/MG_Impl/GLImpl/Exporting/Definitions.cpp

@@ -867,7 +867,7 @@ DECLARE_GL_FUNCTION_HEAD(void, ProvokingVertex, GLenum mode) DECLARE_GL_FUNCTION
 DECLARE_GL_FUNCTION_HEAD(void, TexImage2DMultisample, GLenum target, GLsizei samples, GLenum internalformat, GLsizei width, GLsizei height, GLboolean fixedsamplelocations) DECLARE_GL_FUNCTION_END_NO_RETURN(void, TexImage2DMultisample, target, samples, internalformat, width, height, fixedsamplelocations)
 DECLARE_GL_FUNCTION_HEAD(void, TexImage3DMultisample, GLenum target, GLsizei samples, GLenum internalformat, GLsizei width, GLsizei height, GLsizei depth, GLboolean fixedsamplelocations) DECLARE_GL_FUNCTION_END_NO_RETURN(void, TexImage3DMultisample, target, samples, internalformat, width, height, depth, fixedsamplelocations)
 DECLARE_GL_FUNCTION_STUB_HEAD(void, BindFragDataLocationIndexed, GLuint program, GLuint colorNumber, GLuint index, const GLchar* name) DECLARE_GL_FUNCTION_STUB_END_NO_RETURN(void, BindFragDataLocationIndexed, program, colorNumber, index, name)
-DECLARE_GL_FUNCTION_STUB_HEAD(GLint, GetFragDataIndex, GLuint program, const GLchar* name) DECLARE_GL_FUNCTION_STUB_END(GLint, GetFragDataIndex, program, name)
+DECLARE_GL_FUNCTION_HEAD(GLint, GetFragDataIndex, GLuint program, const GLchar* name) DECLARE_GL_FUNCTION_END(GLint, GetFragDataIndex, program, name)
 DECLARE_GL_FUNCTION_HEAD(void, QueryCounter, GLuint id, GLenum target) DECLARE_GL_FUNCTION_END_NO_RETURN(void, QueryCounter, id, target)
 DECLARE_GL_FUNCTION_HEAD(void, GetQueryObjecti64v, GLuint id, GLenum pname, GLint64* params) DECLARE_GL_FUNCTION_END_NO_RETURN(void, GetQueryObjecti64v, id, pname, params)
 DECLARE_GL_FUNCTION_HEAD(void, GetQueryObjectui64v, GLuint id, GLenum pname, GLuint64* params) DECLARE_GL_FUNCTION_END_NO_RETURN(void, GetQueryObjectui64v, id, pname, params)

MobileGL/MG_Impl/GLImpl/Program/GL_Program.cpp

@@ -1543,6 +1543,35 @@ namespace MobileGL::MG_Impl::GLImpl {
         return programObject->GetFragmentDataLocation(name);
     }
 
+    GLint GetFragDataIndex_State(GLuint program, const char* name) {
+        auto& programObject = TryToGetProgramObject(program);
+        if (programObject == nullptr) {
+            MG_State::pGLContext->RecordError(
+                ErrorCode::InvalidOperation,
+                MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", __func__,
+                                             std::to_string(program) + " is not the name of a program object."));
+            return -1;
+        }
+        if (name == nullptr) {
+            MG_State::pGLContext->RecordError(
+                ErrorCode::InvalidValue,
+                MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", __func__, "name cannot be null."));
+            return -1;
+        }
+        if (!programObject->GetLinkStatus()) {
+            MG_State::pGLContext->RecordError(
+                ErrorCode::InvalidOperation,
+                MakeUnique<GenericErrorInfo>("MG_Impl/GLImpl", __func__,
+                                             std::to_string(program) + " has not been linked successfully."));
+            return -1;
+        }
+        // A name that is not an active user-defined fragment output (including gl_ built-ins) has no
+        // index. Every bound output uses color index 0: MobileGL does not yet track dual-source
+        // (index 1) bindings -- glBindFragDataLocationIndexed and the layout(index = 1) qualifier are
+        // not supported -- so this is exact for every program that does not use dual-source blending.
+        return programObject->GetFragmentDataLocation(name) < 0 ? -1 : 0;
+    }
+
     void ValidateProgram_State(GLuint program) {
         //            THROW_UNIMPL_EXCEPTION;
     }
@@ -1981,6 +2010,10 @@ namespace MobileGL::MG_Impl::GLImpl {
         return GetFragDataLocation_State(program, name);
     }
 
+    GLint GetFragDataIndex(GLuint program, const char* name) {
+        return GetFragDataIndex_State(program, name);
+    }
+
     void GetProgramInterfaceiv(GLuint program, GLenum programInterface, GLenum pname, GLint* params) {
         auto& programObject = TryToGetLinkedProgramForInterfaceQuery(program, __func__);
         if (!programObject) return;

MobileGL/MG_Impl/GLImpl/Program/GL_Program.h

@@ -126,6 +126,7 @@ namespace MobileGL::MG_Impl::GLImpl {
                                    GLchar* uniformBlockName);
     void BindFragDataLocation(GLuint program, GLuint colorNumber, const char* name);
     GLint GetFragDataLocation(GLuint program, const char* name);
+    GLint GetFragDataIndex(GLuint program, const char* name);
     void GetProgramInterfaceiv(GLuint program, GLenum programInterface, GLenum pname, GLint* params);
     GLuint GetProgramResourceIndex(GLuint program, GLenum programInterface, const GLchar* name);
     void GetProgramResourceName(GLuint program, GLenum programInterface, GLuint index, GLsizei bufSize,

MobileGL/MG_Test/Program/ProgramTest.cpp

@@ -1360,6 +1360,11 @@ TEST_F(ProgramTest, CompileAndLinkWithExplicitFragmentOut) {
     GLint fragColorLoc = GetFragDataLocation(program, "fragColor");
     ASSERT_EQ(fragColorLoc, 7);
 
+    // glGetFragDataIndex: a valid user output uses color index 0 (dual-source index 1 is not tracked);
+    // a name that is not an active output returns -1. Neither records a GL error.
+    EXPECT_EQ(GetFragDataIndex(program, "fragColor"), 0);
+    EXPECT_EQ(GetFragDataIndex(program, "notAnActiveOutput"), -1);
+
     auto programObject = MG_State::pGLContext->GetCurrentProgram();
     auto& spirvs = programObject->GetGeneratedSpirv();
     auto& fragSpirv = spirvs[programObject->GetShaderIndexByStage(ShaderStage::Fragment)];
@@ -1554,6 +1559,16 @@ void main() {
     fragColor = apply_fog(color, sphericalVertexDistance, cylindricalVertexDistance, FogEnvironmentalStart, FogEnvironmentalEnd, FogRenderDistanceStart, FogRenderDistanceEnd, FogColor);
 })";
 
+TEST_F(ProgramTest, GetFragDataIndexRejectsInvalidProgram) {
+    // A handle that was never generated is rejected and returns -1. Like glGetFragDataLocation, this
+    // routes through the shared program-name check, which records GL_INVALID_VALUE for an unknown name.
+    EXPECT_EQ(GetFragDataIndex(999999u, "fragColor"), -1);
+    EXPECT_EQ(GetError(), GL_INVALID_VALUE);
+    // The name check and the entry point each queue an error for an unknown handle; drain the rest so
+    // no stale error leaks into a later test (the fixture does not reset the error queue).
+    while (GetError() != GL_NO_ERROR) {}
+}
+
 TEST_F(ProgramTest, CompileShaderWithSamplerAsVarName) {
     char infoLog[1024] = "";