DerivedMesh: minor changes to callback use

- use void instead of float for array arg (cast in the switch statement)
- remove unused args
- use const for poly-mapping and user_data args

source/blender/blenkernel/BKE_DerivedMesh.h

@@ -464,7 +464,8 @@ struct DerivedMesh {
 	                           bool (*setFace)(void *userData, int index), void *userData);
 
 	struct GPUDrawObject *(*gpuObjectNew)(DerivedMesh *dm);
-	void (*copy_gpu_data)(DerivedMesh *dm, int type, float *varray, int *mat_orig_to_new, void *user_data);
+	void (*copy_gpu_data)(DerivedMesh *dm, int type, void *varray_p,
+	                      const int *mat_orig_to_new, const void *user_data);
 
 	/** Release reference to the DerivedMesh. This function decides internally
 	 * if the DerivedMesh will be freed, or cached for later use. */