[cairo] Bug with paint_with_alpha and perf issues whencompositing with SOURCE

[Antoine Azar]

> > That would allow big speedups and correct rendering.   Or is it
> > useless to spend much time and effort on that as SOURCE 
> compositing is
> > not frequently used (and/or no one ever complained about it?) ?   
> 
> Yeah, that too.

If that's the case then I won't waste time to explain why it would be nice
to have a rendering that respects our doc and is not 40x times slower than
it should be. Whenever someone complains that source compositing is broken
then we can revisit the issue.

Antoine