Include new cairo renderer

[27justin]

Hello,

this PR adds a simple cairo renderer as well as a corresponding example to clay, this allows rendering directly into PNGs, PDFs, etc.
I've got it to a point where I believe it to be 'good enough' for now, it notably does not include a proper text shaping engine, instead relying on cairo's toy text API to measure text, etc.
This is flaky for some fonts, though I tweaked it that the result is acceptable for most fonts I tested it against.

Here's a preview of the PDF the example emits:
output.pdf

[nicbarker]

Thanks for all the awesome work here 🎉 just taking a look now!