Zink uses a subset of the u_transfer_helper API that only supports (de)interleaving z/stencil buffers. But doing this currently makes it impossible to use the other features of u_transfer_helper without duplicating their logic. This unifies these code-paths into one, so we can start using other helpers in the future.
I'm not terribly happy with the U_TRANSFER_HELPER_* flag names, especially U_TRANSFER_HELPER_INTERLEAVE_IN_PLACE. But I can't come up with better ones, it seems... Better suggestions are welcome!