Issues with partial repainting when enabling cairo

[Yuri D'Elia]

I briefly tested a build from master with lucid+cairo enabled on X11.
How experimental is "experimental" supposed to be? ;)

It seems that some areas of the frame are not repainted correctly.

For instance, scrolling by pages does not refresh the top (or bottom)
lines at all. More properly, it seems to be a fixed area at the
top/bottom - not really related to visual lines.

Maybe related to 'scroll-margin?

There are some bug reports about cairo on the bts, but nothing so
glaring.