[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/Utils.h

@@ -104,6 +104,13 @@ namespace MobileGL::MG_Backend::DirectGLES {
         String ClampNormFallbackOutputs(String glslCode, GLenum shaderType, Uint32 snormOutputMask,
                                         Uint32 unormOutputMask);
         String ForceFlatIntegerVaryings(const String& glslCode, GLenum shaderType);
+        // Legacy GLSL's gl_FragColor is broadcast to every enabled draw buffer (GL 4.6
+        // 15.2.3), but ShaderSourceProcessor lowers it to the single output mg_FragColor,
+        // which only ever reaches draw buffer 0. Replicates it across `drawBufferCount`
+        // outputs and copies the value into them at the end of main. A no-op for
+        // drawBufferCount <= 1, i.e. for everything but a framebuffer that actually
+        // enables several draw buffers, so the ordinary single-target shader is untouched.
+        String BroadcastLegacyFragColor(String glslCode, GLenum shaderType, Uint drawBufferCount);
         String RemoveLayoutBinding(const String& glslCode);
         // Prefix of the per-sampler float uniform that carries GL_TEXTURE_LOD_BIAS into
         // the shader (see EmulateTextureLodBias); the suffix is the sampler's own name.