Blender Git Loki

Change region drawing callbacks to work much closer to how blender manages them internally.
- yes, this does break scripts, but the api is marked experimental.


ED_region_draw_cb_activate() adds a callback to a region type whereas the api made it look like the callback was being added to the region instance.
Use a class method on bpy.types.Space to manage region drawing, eg.

was:
self._handle = context.region.callback_add(draw_callback_px, args, 'POST_PIXEL')

is now:
self._handle = bpy.types.SpaceView3D.draw_handler_add(draw_callback_px, args, 'WINDOW', 'POST_PIXEL')