[cairo] Render to image buffer artifacts

[Chris]

I've attached the code and image that shows this off.  Scaling at
different levels seems to change the corruption.

For some reason there are artifacts in the alpha channel.  I don't know
if that's the only place, but the alpha channel looks bad.

If you run the code and parse the attached image, directing stdout to a
file, you can see in the lower left corner there are alpha values where
it should be transparent.

Here it is:
00003DFCFFFFFFFFFFFFFFFFFFFFFFFFFFFF
000000C4FFFFFFFFFFFFFFFFFFFFFFFFFFFF
00000013F0FFFFFFFFFFFFFFFFFFFFFFFFFF
0000000055FAFFFFFFFFFFFFFFFFFFFFFFFF
00000000007CFCFFFFFFFFFFFFFFFFFFFFFF
00000000000083F9FFFFFFFFFFFFFFFFFFFF
0000000000000061F1FFFFFFFFFFFFFFFFFF
000000000000000023D1FDFFFFFFFFFFFFFF
0000000000000000000072E0FDFFFFFFFFFF
00000000000000000000000168C8F0FEFFFF
00000001010101020303030303030955A4BE
000000000000000000000000000000000000
000000000000000000000000000000000000

That is the lower corner of the image.  See the values
101010102030303030303 sticking out at the bottom?  That shouldn't be
there because that area is completely transparent.  Sometimes it renders
OK if you change the scaling a little.  Sometimes it looks worse.

Rendering to a PNG file does not seem to cause the same problem.

-- 
// Chris
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