Comment 4 for bug 1248933

This seems to be because transparent layers are set to color 0,0,0,0 (rgba) fading from any color to that will make it go trough grey, because 0,0,0 (rgb) is black. One possible workaround is to fill the transparent layer with a zero alpha white 255,255,255,0 (rgba), note that painting dark colors would then make them "glow" instead.

Maybe cairo can handle all that with the CAIRO_ANTIALIAS_BEST ?
http://www.cairographics.org/manual/cairo-cairo-t.html#cairo-antialias-t