[Pixman] Fast paths for bilinear transformation

[Søren Sandmann]

Hi,

The following patches add some fast paths for bilinear transformation
for the most important formats: a8, r5g6b5, x8r8g8b8, and a8r8g8b8.

The code is actually commented out in the patches. It is enabled in a
bunch of trivial commits here:

      http://cgit.freedesktop.org/~sandmann/pixman/log/?h=more-scalers

that I didn't want to spam the list with.


Soren