Fix out-of-bounds when glTF has many UV sets.
Fix out-of-bounds when glTF has many UV sets.

glTF specifies that client implementations must support at least two UV
texture coordinate sets but does not seem to have an upper limit.

We use a mapping table in order to honor high-numbered UV set indices,
but our mapping table only accomodates 8 entries.

Fixes #2042

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