Reflection maps must be 256x256.
Reflection maps must be 256x256.

This makes the web samples comply with the size requirement in
`filament/src/IndirectLight.cpp`.

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