The cairotextoverlay plugin seems to have a couple of issues:

 (a) text position doesn't seem to work properly

 (b) it seems to do bad memory access somewhere (I managed to reproduce
segfaults, possibly only when using huge font sizes and rendering outside of the
image, not sure)

 (c) the 'font-desc' property isn't actually parsed PangoFontDescription style.
The string is passed as is to cairo_select_font_face() which I believe expects
just a font/family name. The least we want to do is to get the size out of the
font string somewhere.

 (d) the quality isn't that stunning. Maybe we want to convert the rectangle
where cairo renders to to RGB, and let cairo use that as background. Possibly
cairo might be able to do cooler effects that way (better antialising etc.) (I
have no clue, it's just an idea).

Cheers
 -Tim