[Pixman] Valgrind-clean pixman

[Andrea Canciani]

Currently pixman allocates dynamically its implementations, but does
not free them.
This is not a true memory leak, since implementations are only
allocated once, but
many debugging tools will report the asyimmetric malloc/free as a memory leak.
I wrote a patch that avoids allocating pixman_implementation_t's (by
declaring them
to be static) in
http://cgit.freedesktop.org/~ranma42/pixman/commit/?h=wip/valgrind2
More general solutions exists (teardown/reset functions), but
currently they would not
provide additional advantages and would be more complicated.
Objections, suggestions, comments, reviews,... are appreciated.
Andrea Canciani