[Fix, Test] (ShaderTranspiler, DirectGLES): stop printing the default fragment-output colour index into ESSL

MobileGL/MG_Util/ShaderTranspiler/SpvcSession.cpp

@@ -442,6 +442,23 @@ namespace MobileGL {
                 SPVC_CHK_RETURN
             }
 
+            spvc_result SpvcSession::DropDefaultFragmentOutputColorIndex() {
+                if (!(usage & SessionUsageBit::Transpile)) return SPVC_ERROR_INVALID_ARGUMENT;
+
+                SPVC_CHK_INIT
+                const spvc_reflected_resource* list = nullptr;
+                size_t count = 0;
+                SPVC_CHK_RESULT(spvc_resources_get_resource_list_for_type(
+                    resources, SPVC_RESOURCE_TYPE_STAGE_OUTPUT, &list, &count));
+                for (size_t i = 0; i < count; ++i) {
+                    const spvc_reflected_resource& resource = list[i];
+                    if (!spvc_compiler_has_decoration(compiler, resource.id, SpvDecorationIndex)) continue;
+                    if (spvc_compiler_get_decoration(compiler, resource.id, SpvDecorationIndex) != 0u) continue;
+                    spvc_compiler_unset_decoration(compiler, resource.id, SpvDecorationIndex);
+                }
+                SPVC_CHK_RETURN
+            }
+
             spvc_result SpvcSession::Compile(const char** result) {
                 if (!(usage & SessionUsageBit::Transpile)) return SPVC_ERROR_INVALID_ARGUMENT;
                 SPVC_CHK_INIT

MobileGL/MG_Util/ShaderTranspiler/SpvcSession.h

@@ -120,6 +120,25 @@ namespace MobileGL {
                 // `outGlBindings` is appended to, so one vector can collect a whole program's
                 // stages; it may repeat a binding declared by several of them.
                 spvc_result SetAtomicCounterBlockBindings(Int topBinding, Vector<Int>& outGlBindings);
+                // Drops the Index decoration from every fragment output that carries the DEFAULT
+                // colour index 0, so the emitted ESSL does not print `index = 0`.
+                //
+                // Index 0 is what every single-source fragment output already is, in GL and in
+                // ESSL alike, and SPIR-V carries the decoration only because the application
+                // spelled the qualifier out - `layout(location = 0, index = 0) out vec4 c;` is
+                // legal desktop GLSL and says nothing. Printing it back into ESSL is NOT
+                // harmless: GLSL ES has no `index` layout qualifier in core, so the driver
+                // answers "index layout qualifier requires EXT_blend_func_extended" and refuses
+                // the stage. The program then links nothing and every draw with it renders
+                // NOTHING - verified on Mesa 26.1.4 llvmpipe with no MobileGL in the process,
+                // and it is why KHR-GL43.shader_atomic_counters.basic-program-query read back a
+                // black render target.
+                //
+                // A NON-zero index is left exactly as it is: that one really does select the
+                // second dual-source input and cannot be expressed without the extension, so it
+                // must keep reaching the driver (the frontend's own glBindFragDataLocationIndexed
+                // path already emits only non-zero indices for the same reason).
+                spvc_result DropDefaultFragmentOutputColorIndex();
                 spvc_result Compile(const char** result);
                 const SpvcMetadata& GetMetadata() const;
                 const char* GetLastErrorString() const;