Fix GLES3 instanced rendering color and custom data defaults

[bitsawer]

• Fixes GridMap renders 3D meshes as solid black with "Compatibility" renderer #74111
• Fixes Ambient light not affecting gridmap in compatibility renderer #75085
• Fixes Web export fails to display Gridmap that works well in editor and Windows export #80980 (probably, I didn't test a Web build)
• Fixes MultiMesh won't display in web (MultiMeshInstance2D & RenderingServer) #81926 (again probably, but pretty likely)

In the GLES3 Compatibility renderer if a MultiMesh did not have instance colors or custom data enabled, the default instance color and data values ended up being 0.0 (or whatever value happened to be set previously) instead of 1.0 which made most meshes completely black if vertex_color_use_as_albedo flag was enabled on the material. This happens often with GridMap which uses internal MultiMesh objects.

This PR sets default compressed values to 1.0 for color and 0.0 for custom data. Looks like this is also needed in canvas rasterizer to satisfy any buggy. strict or paranoid OpenGL implementations (especially web) that check the input attribute declared data type (uvec4) against the bound type even if the shader branch reading the value is disabled by an uniform condition (FLAGS_INSTANCING_HAS_COLORS and FLAGS_INSTANCING_HAS_CUSTOM_DATA in canvas rasterizer).

[clayjohn]

Looks great!

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.1.3.