gltfio: enable unused UV attributes when possible.
gltfio: enable unused UV attributes when possible.

Reproduced the issue by hacking the TexturedCube glTF file.

Verified the fix by viewing TexturedCube in ubershader mode, then
hacking the shader with matdbg in order to visualize texture
coordinates.

Fixes #2659

google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament,google/filament