patch for background blurring problem

[Henk Boom]

Hi, earlier today I noticed some issues with my backgrounds getting
slightly blurred, and posted about it on the xfce mailing list, here:

http://foo-projects.org/pipermail/xfce/2007-February/020760.html

I looked through the xfdesktop source code, and I think I've found the
culprit. It would seem that bilinear interpolations are not idempotent
even when the source and destinations are the same size, with scales
of 1.0. This is likely (though I don't have enough knowledge of GDK to
be sure) because it takes the edge pixels to be the equivalent of the
edge of the image, rather than the areas 0.5 pixels from the edge. I
added a bit of code to detect when no scaling is done, and switch the
interpolation to nearest neighbour when appropriate. This will
probably have the added bonus of improving speed in these situations.

The attached patch appears to fix the problem for me. Please consider
making use of this patch or an equivalent fix.

Hope this is helpful,
        Henk Boom

P.S. This is the first time I've generated a patch with diff, so if I
should have made it differently please let me know.
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