[cairo] OVER / SOURCE optimization for cairo_paint

[Antoine Azar]

>I was hoping someone else would pick up this topic, but since nobody
>else has: is this really a legitimate optimization when the
>source is scaled or transformed?
>
>  - It will produce clearly incorrect results unless you replace
>    EXTEND_NONE with the (not yet implemented?) EXTEND_PAD, since
>    SOURCE for EXTEND_NONE is going to give 0,0,0,0 pixels outside
>    the edge.

Hi Owen,
You're right about this, and I already fixed this in the latest patch 
I sent to the list. I'd really appreciate it if you had a look at it 
and gave me some feedback.

>In my opinion, optimizations should not change appearance in such a 
>way ... even if the appearance is better .... because it means
>that you get discontinuities between the optimized path and when you 
>can't optimize for one reason or the other.

Agreed. My latest patch shouldn't show any differences, as far as I 
was able to test it.

>The other thing I wonder about the patch in general: what's it going 
>to do for a backend where OVER is normal and SOURCE abnormal? Does the
>backend have to reverse the optimization? (In other words, the 
>question is: should this optimization be in the image backend at a 
>low level or even inside libpixman, rather than in cairo-gstate.c?)

That's an interesting question, I actually didn't think any backend 
could perform faster while doing compositing rather than just 
overwriting the dest with the source pixels. Is it really the case?

Thanks,
Antoine