[Pixman] Improvements to the composite test

[Søren Sandmann Pedersen]

The following patches contain some improvements to the composite
test. 

First, some bugs preventing it from testing a lot of formats are
fixed. This unfortunately has the consequence that it now takes
several hours to run. To fix that, the test is changed so that it only
runs a subset of the tests, determined by a random number generator,
but with a fixed seed so that its output is reproducible.

I also added support for parallelizing it with OpenMP. I'd appreciate
review of this part since I'm not very familar with OpenMP at all.

Finally, it tests a couple more colors, including a superluminescent
one - this is what caught the bug in combine_disjoint_over_u().


Soren