gltfio: Fix "uniform not found" errors.
gltfio: Fix "uniform not found" errors.

The equality operator for MaterialKey was missing some fields (also, its
ordering was inconsistent with the struct), which caused faulty behavior
in the material cache.

I ran into this bug when testing against a Blender-exported model that
has some materials with texture transforms, and some without.

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