[cairo] Operator optimization based on source or dest opacity

[Antoine Azar]

I've taken the advice of various people on the list, and ported this 
optimization from Cairo to Pixman. I've basically checked if / how 
each operator could be optimized if the source and / or dest was 
opaque, and use the optimized operator if possible. The most obvious 
case of this being OVER -> SOURCE for an opaque source. This speeds 
up enormously some common cases, such as a 6.7X improvement in 
painting a RGB source over a 256x256 RGBA dest.

The optimized operators are:
PIXMAN_OP_OVER, PIXMAN_OP_OVER_REVERSE, PIXMAN_OP_IN, 
PIXMAN_OP_IN_REVERSE, PIXMAN_OP_OUT, PIXMAN_OP_OUT_REVERSE, 
PIXMAN_OP_ATOP, PIXMAN_OP_ATOP_REVERSE, PIXMAN_OP_XOR, PIXMAN_OP_SATURATE


One strange thing I noticed: Adding just one boolean check in 
pixman.pict.c's  pixman_image_composite function can have wild 
effects on performance, even though it's not in any critical loop. I 
can't explain it. Adding a if() check to see if the current operator 
could be optimized reduced performance by ~20% for tests that didn't 
switch operators. To counter that effect, I optimized that function 
too by moving some if statements around. The net result is a perf 
increase of up to 35%. Anyone able to explain that phenomenon?

I've benchmarked the most relevant performance cases and posted them here:
http://www.antoineazar.com/optimization_final.htm
Any perf increase above 5% is in bold green and perf decreases below 
5% in bold red

I'm also attaching another patch (to be applied first) that cleans up 
further pixman-compose.c by moving all the composition operators to 
their own files, pixman-compose-operators.c/h

Best,
Antoine
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