[Cairo] Rectangle-based clipping for surfaces

[Carl Worth]

On Oct 15, Keith Packard wrote:
 > I think it's important to keep the intended usage for this operation in
 > mind.  The requirement is that we provide device pixel clipping for things
 > like Expose events.

Yes, this new API is specifically designed for X expose events. I
don't see much use for this specialized pixel-aligned clipping for
offscreen or PostScript surfaces.

 > Alternatively, we could expose a new data type (cairo_region) along with 
 > calls to construct it from rectangles.  I'd like to avoid that as it seems 
 > unnecessary.

I'd also like to avoid this.

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);

Which would prevent muddling of the generic Cairo API.

-Carl