[cairo] problem with downscaling images and destination coordinate spaces

[Vladimir Vukicevic]

The attached test shows a problem with downscaling image surfaces; 
basically, it looks like the valid destination coordinate space shrinks 
down to 32768 / (src_size / (1.0/scale_factor)).  So with a 2048 pixel 
wide source image, scaling down to 16 px wide, anything drawn at an x 
point greater than 32768 / (2048 / 16) = 256 ends up being mangled.

The problem seems to be that pixman transformation matrices are 16.16 
fixed; thus the scaled offsets end up overflowing.  Any ideas what to do 
about this?  This seems much bigger issue than the previous 16.16 issue 
with traps; people generally aren't rendering to targets bigger than 
32kx32k (yet), but people do want to be able to downscale large source 
images...

I think I can work around this on our end by downsampling to a separate 
surface at 0,0 first, and then rendering, but a better fix would be helpful.

    - Vlad

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