LoadModelAnimations takes an `int` for the animation count parameter.
The animation count should never be negative, so it makes sense to
specify it as unsigned in the API. This matches the API for
UnloadModelAnimations, which expects an unsigned int. Both GLTF and IQMM
also store the animation count internally as unsigned, and we were
casting to a signed int for no reason.
GLTF actually uses `size_t` internally, so we're technically risking
overflow, but having 2^32 animations seems unlikely.
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