[Pixman] Bilinear scaling patches for MMX

[Matt Turner]

These five patches implement the same bilinear scaling compositing functions
as provided by the SSE2 code. They pass the test suite on x86, Loongson, and
iwMMXt, but I haven't done extensive benchmarking yet on iwMMXt.

The fifth patch optimizes the functions for 7-bit bilinear interpolation, but
doesn't give the performance differences I would expect. firefox-planet-gnome
performance is increased by ~1% and firefox-fishtank performance is reduced.

Matt