Add instructions for running with cairo

[dianarg]

• Related to Provide install instructions for cairo-rs dependency which requires provided cairo dll #9
• Related to Unresolved import direct2d::math #33

[dianarg]

Hi everyone,
I tested this on Windows 10 and Ubuntu Linux. Getting the cairo library on Windows was slightly more complicated, but the release rylev mentioned worked fine for me (https://github.com/preshing/cairo-windows).

[cmyr]

What's the significance of this change?

edit: oh, maybe #33 (comment)?

[dianarg]

Yes, that's where it came from.

[dianarg]

Perhaps it also makes sense to remove piet-cairo from this list and have users explicitly choose a backend with --features.

[cmyr]

Thanks for this, cleanup is good and documentation is even better. One thing I would like to make sure is that this doesn't influence what gets built in CI; additionally we might want to also be building cairo in CI on windows? I'm not sure how much of a headache that would be.

[dianarg]

I haven't used Azure before, but it looks like what runs in CI is controlled by ci/test.yml? Since the commands already have --all, I think it's already building everything (except cairo) for Windows.

piet/ci/test.yml

Line 16 in 02207a9

cargo +${{parameters.toolchain}} check --all --exclude piet-cairo || exit

For adding cairo, what do you think about just adding a curl line or similar to install-rust.yml to download the prebuilt release listed above?

[cmyr]

okay, --all sounds good. And yea, we would do something like download prebuilt binaries. I don't think that's critical though, and it doesn't need to be part of this PR; if you want to play around with it you're more than welcome but no pressure. :)