Speed up bilinear interpolation.

Speed up bilinear interpolation.

Speed up bilinear interpolation by processing more than one component
at a time on 64 bit architectures, and by precomputing the dist{ixiy}
products on 32 bit architectures.

Previously bilinear interpolation for one pixel would take 24
multiplications. With this improvement it takes 12 on 64 bit, and 20
on 32 bit.

This is a small but consistent speedup on the swfdec-youtube
benchmark:

[ # ]  backend                         test   min(s) median(s) stddev. count
Before:
[  0]    image               swfdec-youtube   18.010   18.020   0.09%    4/5

After:
[  0]    image               swfdec-youtube   17.488   17.584   0.22%    5/6

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