#6 Patch to cope with alpha

[Jasper]

If found that perceptualdiff did not cope well when comparing two PNGs with a (completely) transparent background if the background color was different. This should have no effect as the difference will never be visible (the color has an associated alpha of 0).

I used the attached patch to solve this problem (simply blends with a background).

Originally I made this patch against 1.0.2, and I've just modified it a bit to suit trunk. However, I saw that the description of RGBAImage now said that premultiplied alpha was assumed. If this is indeed correct and perceptualdiff does not exhibit this problem anymore, then this patch can be disregarded.

[Jasper]