[cairo] Bug with paint_with_alpha and perf issues when compositing with SOURCE

[Antoine Azar]

> Maybe the phrasing of "the effect is similar to" 
> just needs to be "the effect (with the default OVER operator) 
> is similar to" in order to be accurate?

Well that's not really an "accurate" description of what the function does
either :-)

If the current result is really what Cairo users want, then the obvious
optimization is to replace the SOURCE operator by OVER for this function. We
get an instant 10X speedup in pixman and 5X speedup in Quartz for free. In
any case, I don't really understand why we're substituting SOURCE and CLEAR
by a complicated string of operators. If the goal is to use
backend-accelerated functions instead of going the pixman route, we should
maybe revisit that decision as I'm pretty sure Pixman can do it faster
natively (which was maybe not the case at the time this decision was
taken?).

Antoine