The asset previously had three confusing fields related to textures:
mTextures, mTextureSlots, and mTextureBindings.
These are now consolidated into a single field, which simply has one top
level array item per `cgltf_texture`.
The memory footprint is smaller because we no longer bother to store
`TextureSampler` objects, instead we simply defer their construction
until calling setParameter.
Last but not least, we now assert in debug builds if the usages of a
particular texture have inconsistent sRGB flags. In the past,
inconsistent sRGB would be silently ignored.
In other words, we will now assert if you try to use the same texture
for `baseColor` and `normalMap`, whereas in the past we would simply
exhibit non-deterministic behavior in this situation (with respect to
sRGB semantics).
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