[cairo] Better quality downsampling in cairo/pixman

[Bill Spitzak]

Cairo currently does only bilinear interpolation.

This is wrong for any scale less than 1, although the artifacts do not 
really appear until a scale less that .5. Bilinear cannot use more than 
4 adjacent pixels in a square to produce an output pixel, and if you 
scale small enough the destination pixel will cover far more than 4 
pixels in the source.

I think everybody is in agreement that this needs to be fixed.