Support for compressed sRGB formats

[mvaligursky]

• Both WebGL and WebGPU supports sRGB version of compressed texture formats
• compressed texture formats are used when the asset has { srgb = true; } option, similarly to non-compressed formats
• glb parser requests srgb texture formats for base and emissive color textures, as those are stored in sRGB space
• updated couple of fragment shaders in the examples: as glb supplied diffuse texture is not stored in sRGB format, which is automatically sampled as a linear value, the custom shader now needs to convert the result back to sRGB. The shader can no longer work completely in sRGB space.

partially implements #3715

[LeXXik]

By the way, is there a decode function for 4 component vectors? I remember we needed a texture with alpha channel, but seems $DECODE variants are only for vec3. Not a big deal, as we just manually do the same, just curious.

[mvaligursky]

We don't have one as transparency is in linear space. But I guess if you store other data in the alpha channel (say a greyscale color), you might need to use a custom decode function.

[LeXXik]

Right, custom data. Alright, thanks.

[slimbuck]

I still think it would be neater to flag the few hdr formats as hdr rather than the relatively many formats as ldr. (Integer formats aren't color formats and so shouldn't get either flag).

[slimbuck]

Approved with some comments