Bug 12801

Hello, this is a patch to pixman that optimize the
PIXMAN_FILTER_NEAREST + PIXMAN_REPEAT_NORMAL + n_clip == 1 case. It
turns out that this case is the fastest one when scaling images with
cairo without rotating.

On my computer, this speeds up pure non-rotating scales of ARGB
surfaces where source is 64x64 and destination size is 1000x1000 from
49.60 ms to 13.20 ms. I have also attached a test program which is
supposed to emulate GDK's gdk_pixbuf_color_composite() function with
cairo. Using that program the speed up is from 62.08 ms to 31.88 ms.

The downside is that the patch 153 lines of code. 

I think that all cases should be expanded in a similar way. If the
for-statement that plot pixels contain an if-statement, then that
if-statement should be moved out of the loop if possible. This
increases the code size by a lot, but that should be possible to clean
up using clever macros. Kind of like how the pixops library does it.