[cairo] Faster bilinear scaling

[Soeren Sandmann]

Hi,

This branch:

        http://cgit.freedesktop.org/~sandmann/pixman/log/?h=bilinear

contains a fast path for fetching of bilinearly filtered, scaled
images. It is basically Andre's work, described here:

        http://lists.cairographics.org/archives/cairo/2008-December/016170.html

What I did was

        - Update scaling-test to also test bilinear scaling

        - Remove bilinear_interpolation_left/right() functions in
          favor of just calling bilinear_interpolation().

        - Fix coding style.

The performance improvement for the swfdec-youtube benchmark on a
3.8GHz P4 is around 17%:

Before:

[ # ]  backend                         test   min(s) median(s)  stddev. count
[  0]    image               swfdec-youtube    8.375    8.431   0.44%   6/6

After:

[ # ]  backend                         test   min(s) median(s) stddev. count
[  0]    image               swfdec-youtube    6.942    7.019   0.61%  6/6

Much of the profile of this benchmark is in radial gradients, so other
users of bilinear scaling may see more improvement.

Also, if anyone is interested in adding support for SIMD acceeleration
of fetchers, the the bilinear_interpolation() function is begging to
be written with SSE2 or NEON.

Comments welcome.


Thanks,
Soren