[cairo] Alpha problems at some resolutions

[John Perkins]

I have been seeing a problem with certain image sizes first in 1.4.10 
and now in 1.6.4 using several different in-house test applications 
written by two different people.

The attached png should be white text on a fully transparent background.

At most resolutions, the transparent area around the text is partially 
transparent yellow.  It is never another color, the problem is very 
consistent.

If I render into a 640x480 image using otherwise identical code, I get 
the correct output.  However, 640x85 has the yellow problem.  This leads 
me to believe that stride alone is not a factor.

This was output by cairo win32 using the toy text functions into a 
178x85 image surface.  Other tests have used circles or rectangles to 
rule out problems with the text functions.

Regardless of what is used to save or display the image the problem 
still exists.

Any suggestions would be greatly appreciated.

Thanks,
John
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