[Feat] (MobileGL): full dual-source blending across state, transpiler, and both backends

Wire GL_SRC1_* dual-source blend factors (glBlendFunc) end to end with the
glBindFragDataLocationIndexed color index, so a fragment shader can drive both
dual-source blend inputs.

State + converters:
  - RenderState BlendFactor gains Src1Color/OneMinusSrc1Color/Src1Alpha/
    OneMinusSrc1Alpha; GLToMG/MGToGL/MGToVk/MGToStr converters map them to
    GL_SRC1_*, VK_BLEND_FACTOR_SRC1_*, and readable names.

Transpiler layout(index = N):
  - ProgramAttrib carries explicitFragmentOutIndices; ProgramObject threads
    m_explicitFragDataIndex into it at both link sites.
  - TMglGlslIoResolver applies the color index as TQualifier.layoutIndex on the
    fragment output, emitting layout(index = 1) via the glslang Index decoration
    -> SPIRV-Cross path. Only the non-zero (dual-source) index is emitted: index 0
    is the GL default and an explicit "index = 0" would demand
    GL_EXT_blend_func_extended on GLES for ordinary single-source outputs.

Feature detection, POST, and hard-fail at use time (no silent fallback):
  - Vulkan: dualSrcBlend is detected at device creation and cached; a draw whose
    enabled blend state uses a SRC1 factor without the feature throws at pipeline
    build with the reason and a pointer to the POST row.
  - GLES: GL_EXT_blend_func_extended detected at load into
    GLESCapabilities.SupportsDualSourceBlend; a draw enabling blend with a SRC1
    factor without it throws in the blend-state sync with the same guidance.
  - DriverPost adds a dual-source-blend row for both backends (Pass/Warn).

Tests:
  - ProgramTest.CompileAndLinkWithExplicitFragmentOut now asserts the transpiled
    fragment shader carries layout(location = 0, index = 1) after a re-link with
    glBindFragDataLocationIndexed(index 1), and still omits any index qualifier
    for the plain index-0 output.

MobileGL/MG_Util/ShaderTranspiler/ShaderCompiler.cpp

@@ -205,6 +205,7 @@ namespace MobileGL {
                     resolver =
                         MakeUnique<TMglGlslIoResolver>(*program, (EShLanguage)stage, attrib.explicitVertexInLocations,
                                                        attrib.explicitFragmentOutLocations,
+                                                       attrib.explicitFragmentOutIndices,
                                                        attrib.explicitOpaqueUniformBindings);
                     break;
                 }

MobileGL/MG_Util/ShaderTranspiler/Types.h

@@ -31,6 +31,9 @@ namespace MobileGL {
                 Vector<SharedPtr<glslang::TShader>> shaders;
                 UnorderedMap<String, Uint> explicitVertexInLocations;
                 UnorderedMap<String, Uint> explicitFragmentOutLocations;
+                // Dual-source blend color index per fragment output (glBindFragDataLocationIndexed) ->
+                // emitted as layout(index = N).
+                UnorderedMap<String, Uint> explicitFragmentOutIndices;
                 UnorderedMap<String, Uint>* explicitOpaqueUniformBindings = nullptr;
             };
 

MobileGL/MG_Util/ShaderTranspiler/glslang/TMglGlslIoResolver.cpp

@@ -76,6 +76,15 @@ namespace MobileGL {
                 auto& writableType = ent.symbol->getWritableType();
                 writableType.getQualifier().layoutLocation = it->second;
             }
+            // Dual-source blend color index (glBindFragDataLocationIndexed) -> layout(index = N).
+            // Only the non-zero (dual-source) index is emitted: index 0 is the GL default, and
+            // emitting an explicit "index = 0" qualifier would demand GL_EXT_blend_func_extended on
+            // GLES even for ordinary single-source fragment outputs.
+            auto idxIt = m_explicitFragOutIndices.find(name.c_str());
+            if (idxIt != m_explicitFragOutIndices.end() && idxIt->second != 0) {
+                auto& writableType = ent.symbol->getWritableType();
+                writableType.getQualifier().layoutIndex = idxIt->second;
+            }
         }
         if (ShouldAssignPlainUniformLocation(type)) {
             const int size = glslang::TIntermediate::computeTypeUniformLocationSize(type);

MobileGL/MG_Util/ShaderTranspiler/glslang/TMglGlslIoResolver.h

@@ -26,13 +26,15 @@ namespace MobileGL {
     public:
         using ExplicitVarSlotMap = UnorderedMap<String, Uint>;
         TMglGlslIoResolver(const glslang::TIntermediate& intermediate, const ExplicitVarSlotMap& vertexIns,
-                           const ExplicitVarSlotMap& fragOuts, ExplicitVarSlotMap* opaqueUniformBindings)
+                           const ExplicitVarSlotMap& fragOuts, const ExplicitVarSlotMap& fragOutIndices,
+                           ExplicitVarSlotMap* opaqueUniformBindings)
             : TDefaultGlslIoResolver(intermediate), m_explicitVertexIns(vertexIns), m_explicitFragOuts(fragOuts),
-              m_explicitOpaqueUniformBindings(opaqueUniformBindings) {}
+              m_explicitFragOutIndices(fragOutIndices), m_explicitOpaqueUniformBindings(opaqueUniformBindings) {}
         TMglGlslIoResolver(const glslang::TProgram& program, const EShLanguage stage,
                            const ExplicitVarSlotMap& vertexIns, const ExplicitVarSlotMap& fragOuts,
-                           ExplicitVarSlotMap* opaqueUniformBindings)
-            : TMglGlslIoResolver(*program.getIntermediate(stage), vertexIns, fragOuts, opaqueUniformBindings) {}
+                           const ExplicitVarSlotMap& fragOutIndices, ExplicitVarSlotMap* opaqueUniformBindings)
+            : TMglGlslIoResolver(*program.getIntermediate(stage), vertexIns, fragOuts, fragOutIndices,
+                                 opaqueUniformBindings) {}
         void reserverStorageSlot(glslang::TVarEntryInfo& ent, TInfoSink& infoSink) override;
         void reserverResourceSlot(glslang::TVarEntryInfo& ent, TInfoSink& infoSink) override;
         int resolveUniformLocation(EShLanguage stage, glslang::TVarEntryInfo& ent) override;
@@ -43,6 +45,7 @@ namespace MobileGL {
 
         const ExplicitVarSlotMap& m_explicitVertexIns;
         const ExplicitVarSlotMap& m_explicitFragOuts;
+        const ExplicitVarSlotMap& m_explicitFragOutIndices;
         ExplicitVarSlotMap* m_explicitOpaqueUniformBindings = nullptr;
         std::map<glslang::TString, int> m_plainUniformLocationSizeByName;
         std::map<glslang::TString, int> m_plainUniformLocationByName;