[cairo] 'erasing' a surface

[Jonathan Roewen]

Hi,

I've been running myself around in circles trying to figure out how to
effectively erase a surface.

Basically, I want to take a cairo object, and draw an opaque black
rectangle over a given dimension.

I'm using the OCaml bindings, and every which way I've tried it so far
doesn't work. Either I get this massive white rectangle that doesn't
fit with anything I've coded, or whatever I do doesn't have any
noticable effect.

Jonathan