Raise sampler limit to 16.
Raise sampler limit to 16.

In practice this limits materials to 10 samplers since Filament uses 5
for lights and skips 1 slot for the post-process sampler. This can
probably be optimized.

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