[cairo] Cairo Gotchas Notebook

[Lawrence D'Oliveiro]

I have added a new notebook, titled “Cairo Gotchas”, to my Qahirah
Notebooks collection <https://github.com/ldo/qahirah_notebooks>.

The main real gotcha is the surprising results you get when trying to
determine the extents of a path while some non-identity CTM is in
effect (as discussed recently on this list). I also mention the
behaviour of the pattern-context matrix--that one is not a bug, it’s a
feature :).

Suggestions for further additions welcome. Is there anything about the
behaviour of Cairo that surprised you?