Support packed textures of different color spaces

[UX3D-haertl]

This is a low priority issue.
Textures in glTF can be reused in any material property which expects textures. This means, that the same texture might be used for different purposes with different color spaces, e.g. as baseColorTexture and occlusionTexture.
In this case we need to upload the texture to the gpu twice, once with RGBA format and once with sRGBA format.
Currently, only texture packing in the same color space/format is supported.