[cairo] [PATCH] Pixman uses separable convolutions for GOOD/BEST

[Bill Spitzak]

This replicates the results of my recent cairo patch in pixman.
GOOD/BEST will use larger filters for scaling down to produce
nice resulting images. BEST will also produce "square" pixels
when scaling up, useful for image editors and similar to OS/X.

With these changes, provided they are also placed into the xrender
back end, the cairo patch for filtering can be removed.