Run examples on CI on Windows / Mac / Linux

[almarklein]

I think on CI we now only run stuff on Linux with LavaPipe. But it should now also work on MacOS, see gfx-rs/wgpu-native#364. Windows should also work, but IIRC not all our tests pass for DX12 yet.

Would be good to:

• Run tests of wgpu-py on all possible platforms.
• Have insight into what platform is fastest (since they're emulated GPU's).
• Use the faster variant for e.g. pygfx's validation examples and docs.