Dedicated test for correct pixel transfer functions

As of today, only four glTF color textures use sRGB encoding:
* `emissiveTexture`
* `pbrMetallicRoughness.baseColorTexture`
* `KHR_materials_pbrSpecularGlossiness.specularGlossinessTexture`
* `KHR_materials_sheen.sheenColorTexture`

All other textures are sampled linearly. We need dedicated test models that would highlight potential implementation errors in that area.

It's important to note, that working with linear 8-bit values in DCC tools could be quite challenging as some editors may misrepresent such content on screen and show misleading color-picker values.

Start by reviewing the four listed sRGB texture uses: emissiveTexture, baseColorTexture, specularGlossinessTexture, and sheenColorTexture, then compare them with the other texture types described as linear. Create dedicated test models whose colors expose incorrect transfer-function handling. Done means the models cover these encoding cases and make implementation errors distinguishable.

Written by the indexing model from the issue text.