[cairo] cairo-gl and interoperability

[Eric Anholt]

Here's a pair of patches to hopefully bring cairo-gl towards more
general usability: Explaining how to have cairo-gl interact with other
GL usage and with threading.  It also brings cairo-gl's performance
back to where it was, so we're not (approximately) tied with
cairo-image on the cairo-traces benchmarks.

For viability of the backend, we could use some demos of interop using
context sharing, and some testcases of it plus testcases of threading
if possible.