Re-enable clipping for cairo backend

Here's a second[*] patch for the cairo backend.

This one re-enables clipping. All the necessary code was present
already, but disabled with a comment claiming problems on two of the
examples. I double-checked both examples but found no problems,
(whereas, obviously without clipping things don't work at all after


[*] Apparently my first patch hit the list before my subscription
request was completely processed. So it's either been queued up for
moderation or deleted by now. Please let me know if I should re-send

0001-cairo-Re-enable-clipping.patch (1.06 KB)