In some cases transform modes would use the custom-data pointer,
other times the transform conversion functions would.
However with some combinations (bone mirror + bend for eg),
both conversion & transform mode would use this pointer causing a crash.
Fix this by having 2 custom-data pointers:
one for the mode, another for the data-type.
This also simplifies time-slide which was conditionally mixing mode/type data in the one array.
c2508b0aaf