pixman-fast-path.c: Pick NEAREST affine fast paths before BILINEAR ones

When a BILINEAR filter is reduced to NEAREST, it is possible for both
types of fast paths to run; in this case, the NEAREST ones should be
preferred as that is the simpler filter.

Signed-off-by: Soren Sandmann <soren.sandmann@gmail.com>
Reviewed-by: Bill Spitzak <spitzak@gmail.com>