[cairo] is self-copy supposed to work?

[Benjamin Otte]

On Fri, 2010-04-30 at 12:28 +0100, Chris Wilson wrote:
> Currently self-copy behaviour is underspecified. I think the consensus is
> that self-modification is undesired...
> 
Last I brought this up the consensus was a lot stronger than just
"undesired". 
I'd vote to make it an explicit error on the gstate if you set a
source/mask that was identical to the target.
If you document that clearly for set_source() and mask() and enable
using push_group() as a workaround, the problem would almost be solved.

I also suppose that a lot of people would expect self-copies to do
special magic (like XCopyArea), and I think trying to provide that is
both a lot of work and often impossible anyway.

Benjamin