Implemented in 156d0c3b4291e685c09b3f31222c6acdaa10c68f in a different way.

drawdata is now a Listbase, so in the future it's easier to extend the drawing system to beyond stereo (e.g., fulldome).

I'm storing one only wmDrawTriple per wmDrawData struct. It may be better to store two (triple_data and triple_add) to avoid the joggling with the ids as I'm doing.