[cairo] Operator optimization based on source or dest opacity

[Soeren Sandmann]

Hi Antoine, 

> I'm attaching the three new patches. Let me know how it looks now.

The refactoring one looks great, feel free to push. One small thing is
that all the combiner functions could be made static, but I'll leave
it up to you whether to do that.

As I said in an off-list mail, I'll take a look at the operator
replacement either Thursday or Friday.


Thanks,
Soren