Comment 6 for bug 512615

It looks like the culprit is in cairo-win32-font.c, judging from these comments (line 1186):

"* Otherwise, we need to draw using software fallbacks. We create a mask
 * surface by drawing the the glyphs onto a DIB, black-on-white then
 * inverting. GDI outputs gamma-corrected images so inverted black-on-white
 * is very different from white-on-black. We favor the more common
 * case where the final output is dark-on-light."

After that (at line 1212):

"* For ClearType, we need a 4-channel mask. If we are compositing on
 * a surface with alpha, we need to compute the alpha channel of
 * the mask (we just copy the green channel). But for a destination
 * surface without alpha the alpha channel of the mask is ignored"
(Let's not forget that our surface is semi-transparent, so it does have an alpha!)

And a few lines later (line 1225):

"* XXX: Hacky, should expose this in cairo_image_surface"

Probably this is a bug in Cairo itself, but it seems to need quite a major rewrite.