Retrieves the index of the clump vertex with which the user-draw is aligned.
userdraw Pointer to the user-draw.
The index of the vertex the user-draw is aligned with if successful, and 0 otherwise.
The vertex index is only used if the user-draws type is rwVERTEXALIGN, for all other user-draw types it is ignored.
The vertex index is an index into the vertex list of the owning clump of the user-draw.