WebGL: fix isTextureFormatSupported for ETC2 formats.
WebGL: fix isTextureFormatSupported for ETC2 formats.

It was assumed that ETC2 is supported on all ES3 devices, but WebGL on
desktop machines does not support ETC2.

Note that WebGL has its own special extension strings for compressed
textures.

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