I know that an expose event contains a region, but this does not help,
we use the draw event, and I don't know how to reliably access the
region. More, it is not really easy to determine what needs to be
redrawn from inside a draw event.
The cairo context you get in the draw signal is clipped to the expose region by default, which means that cairo will do culling for you automatically. If you need to do extra processing to determine if some complex update method should be run based on the clip region (which is really a big edge case -- you shouldn't need to do this!), you can use cairo_in_clip or cairo_clip_extents.
