Use unsigned int for animation count. (#2002)

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.

src/raylib.h

@@ -1436,7 +1436,7 @@ RLAPI void SetMaterialTexture(Material *material, int mapType, Texture2D texture
 RLAPI void SetModelMeshMaterial(Model *model, int meshId, int materialId);                  // Set material for a mesh
 
 // Model animations loading/unloading functions
-RLAPI ModelAnimation *LoadModelAnimations(const char *fileName, int *animCount);           // Load model animations from file
+RLAPI ModelAnimation *LoadModelAnimations(const char *fileName, unsigned int *animCount);   // Load model animations from file
 RLAPI void UpdateModelAnimation(Model model, ModelAnimation anim, int frame);               // Update model animation pose
 RLAPI void UnloadModelAnimation(ModelAnimation anim);                                       // Unload animation data
 RLAPI void UnloadModelAnimations(ModelAnimation* animations, unsigned int count);           // Unload animation array data