Add fast path scaled, bilinear fetcher.

Add fast path scaled, bilinear fetcher.

This adds a bilinear fetcher for the case where the image has a scaled
transformation, does not repeat, and the format {ax}8r8g8b8.

Results for the swfdec-youtube benchmark

Before:

[ # ]  backend                         test   min(s) median(s) stddev. count
[  0]    image               swfdec-youtube    7.841    7.915   0.72%    6/6

After:

[ # ]  backend                         test   min(s) median(s) stddev. count
[  0]    image               swfdec-youtube    6.677    6.780   0.94%    6/6

These results were measured on a faster machine than the ones in the
previous commit, so the numbers are not comparable.

Signed-off-by: Søren Sandmann Pedersen <sandmann@redhat.com>