[Cairo] Rectangle-based clipping for surfaces

[Keith Packard]

Around 11 o'clock on Oct 15, Bill Spitzak wrote:

> My personal feeling is that the only way to increase the clip region should 
> be a cairo_restore call. Can you instead use a single call that takes a list 
> of rectangles? Then that call can union the rectangles and then intersect 
> with the current clip.

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.  In the Expose event case, we need to incrementally 
construct the clip list from a sequence of expose events, once the set is 
complete, we want to render everything restricted to that clip list.  The 
proposed API (clip_begin/clip_rectangle) precisely matches this usage.

We could also have an 'clip_end' call and say that the accumulated 
rectangles are intersected with the current pixel clip at that point; 
that's actually necessary for X operations in any case.

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.

-keith