[Fix] (DirectGLES): broadcast legacy gl_FragColor to every draw buffer

Legacy GLSL's gl_FragColor goes to every enabled draw buffer (GL 4.6 15.2.3),
but ShaderSourceProcessor lowers it to a single mg_FragColor output, which only
ever reaches draw buffer 0. Everything past the first attachment kept its
pre-draw contents.

Replicated across the enabled draw buffers with copies at the end of main.
Gated on the count so the ordinary single-target shader is byte-for-byte what it
was: the pass is a no-op below two draw buffers, and the count comes from the
frontend draw framebuffer at program-sync time (not from the backend framebuffer
sync, which only runs later in PrepareForDraw - a program compiled against a
stale count would not be relinked until the draw after the one that needed it).
It joins the snorm/unorm clamp masks as framebuffer state the shader is compiled
against, with the same relink-on-change check.

Also advertises GL_ARB_explicit_attrib_location and GL_ARB_texture_multisample,
which DirectGLES implements for every version it advertises but only listed for
DirectVulkan. Both are core from GL 3.2/3.3 on, so an app targeting 3.0/3.1
reaches them only through the extension string - without the former the CTS
picks an entirely different draw_buffers shader, and without the latter
KHR-GL31.texture_size_promotion.functional crashed outright.

KHR-GL3{0,1,2,3}.draw_buffers.draw_buffers_1 now passes on all four versions,
and texture_size_promotion.functional on GL31 downgrades from a crash to a
(still open) comparison failure.

MobileGL/MG_Backend/DirectGLES/Managers.cpp

@@ -3181,6 +3181,7 @@ namespace MobileGL::MG_Backend::DirectGLES {
 
     namespace PrgramImpl {
         Uint32 g_snormFallbackClampOutputMask = 0;
+        Uint g_fragColorBroadcastCount = 1;
         Uint32 g_unormFallbackClampOutputMask = 0;
         Uint g_lastUsedBackendProgramId = 0;
         StateBackendObjectRegistry<MG_State::GLState::ProgramObject, BackendProgramObjectImpl> g_backendProgramObjects;
@@ -3234,6 +3235,7 @@ namespace MobileGL::MG_Backend::DirectGLES {
                     stateProgramObject->GetExternalIndex(), m_backendProgramId);
             m_snormFallbackClampOutputMask = g_snormFallbackClampOutputMask;
             m_unormFallbackClampOutputMask = g_unormFallbackClampOutputMask;
+            m_fragColorBroadcastCount = g_fragColorBroadcastCount;
 
             // Detach all existing shaders
             GLint attachedCount = 0;
@@ -3347,6 +3349,7 @@ namespace MobileGL::MG_Backend::DirectGLES {
                 source = RemoveLayoutBinding(source);
                 source = ProcessOutColorLocations(source);
                 source = ForceFlatIntegerVaryings(source, glShaderType);
+                source = BroadcastLegacyFragColor(std::move(source), glShaderType, m_fragColorBroadcastCount);
                 source = EmulateTextureLodBias(source);
                 source = EmulateBaseInstanceInVertexShader(std::move(source), glShaderType);
                 source = PromoteDrawParameterGlobalsToUniforms(std::move(source), glShaderType);