Shader compilation in GitHub Actions

[raphlinus]

This patch sets up very basic CI (right now just cargo fmt) but more importantly compiles shaders in a GitHub Action.

Any PR to branches other than main will run shader compilation. Any push to the dev branch will run shader compilation and then merge to main.

Closes #177

[raphlinus]

The CI failed for a technical reason (it doesn't run shader compilation on main but the PR is against master; I want to do the rename but haven't yet). Possibly I could do more fine-tuning to make this more robust, or in any case produce better error messages.

[dfrg]

Adding -force here should suppress the error if the directory already exists. It might be worth not doing this though and just rejecting any PRs that contain the gen directory?

[dfrg]

I'm not the most qualified to review GitHub actions, but nothing here stands out as problematic to me.

I assume that after this lands, all PRs should be directed to dev?

[raphlinus]

Yes. I think something that should happen in this PR is that the README gets updated with clear language describing what's going on. People will want main if they're building, but dev if they're either making updates or wanting to look at clean PRs without the noise of generated compiled shaders. There's obviously a ton more that can be done in CI as well, but my current thinking is to land this with minimal changes (aside from the README) and add more as followup PRs.

One other question is that we have a few more gen directories (tests, examples for piet-gpu-hal). Should we do all of these so it's consistent?

[dfrg]

This all looks good to me. Now seems as good a time as any to make the change.