[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/BackendObject_DirectGLES.cpp

@@ -832,6 +832,12 @@ namespace MobileGL::MG_Backend::DirectGLES {
                                           E_GL_ARB_multi_draw_indirect, E_GL_ARB_indirect_parameters,
                                           E_GL_ARB_shader_draw_parameters, E_GL_ARB_gpu_shader5, E_GL_ARB_multi_bind,
                                           E_GL_ARB_shading_language_420pack, E_GL_ARB_vertex_attrib_binding,
+                                          // Both are core from GL 3.2/3.3 on and implemented here for
+                                          // every advertised version, but an app targeting 3.0/3.1
+                                          // only reaches them through the extension string - the CTS
+                                          // picks a whole different shader for draw_buffers without
+                                          // explicit_attrib_location. DirectVulkan advertises both.
+                                          E_GL_ARB_explicit_attrib_location, E_GL_ARB_texture_multisample,
                                           E_GL_ARB_shader_image_size};
         // Only advertised when the device driver actually has usable timer queries
         // (GL_EXT_disjoint_timer_query plus its entry points) and the