[Cairo] Rectangle-based clipping for surfaces

[Thomas Hunger]

> Instead, if we restrict the new API to the Xlib-specific backend then
> it can just use the existing Xlib Region object:
>
> void
> cairo_xlib_surface_set_clip_region (cairo_surface_t *surface,
> 				    Region region);

This function is useful if there is an external expose_event (because a part 
of the window was exposed or similar). But what if I draw some stuff on a 
surface and want to have the exact pixel size of the bounding box,  for 
example to raise an expose_event in the first place (mark an area dirty)? 
This could be useful for animation, but also for saving images which are no 
larger than the actual cairo output (no cropping would be required 
afterwards).
Also the geometrically exact bounding box could be interesting for some 
programs.
Is there currently a function or way in cairo to get this data?
Tom