cu found a bug when using clipping and fills with the image backend, but
it turns out to be the lost sign bug afflicting everything...
To trigger the bug requires evaluating the clip surface prior to using
in with _cairo_clip_combine_with_surface(). Which is taken along a
particular path when using a clip mask with non-pixel geometry on image,
but more easily hit by the fallback code.
Reported-by: <cairouser@yahoo.com>
