Add WebAssembly support

[flupke]

What changed

• add support for the wasm32-unknown-emscripten target
• add an example to run it

Note it's the very first time I write Rust code, please don't hesitate to educate me in this review :)

Why?

To fix #39 and because we needed this for https://jitter.video/ (rebuilding our rendering engine in Rust).

How to test

Follow the instructions I added in the top-level README and in wasm-example/ (not sure this is the right location).

[ctrlcctrlv]

Fantastic news. Sorry to bother but do you think it can compile https://github.com/MFEK/glif/? That's also a way to test the target. Emscripten has SDL2 native support, why I chose it.

[flupke]

Fantastic news. Sorry to bother but do you think it can compile https://github.com/MFEK/glif/? That's also a way to test the target. Emscripten has SDL2 native support, why I chose it.

It seems to use skia-safe so... maybe? Though it seems to come with other C dependencies like imgui, so they will need to support wasm32-unknown-emscripten too.

[flupke]

Fixed a typo in 70ab149. This is a fixup commit, if this PR is approved, please give me some time to cleanup the branch before merging it 🙏

[pragmatrix]

Excellent. I got the example running on both ubuntu and macOS. Some notes:

• In wasm-example/ make build I get the following repetitive warnings on macOS and Ubuntu:

warning: em++: warning: linker setting ignored during compilation: 'EXPORTED_RUNTIME_METHODS' [-Wunused-command-line-argument]
warning: em++: warning: linker setting ignored during compilation: 'ERROR_ON_UNDEFINED_SYMBOLS' [-Wunused-command-line-argument]
warning: em++: warning: linker setting ignored during compilation: 'MAX_WEBGL_VERSION' [-Wunused-command-line-argument]
warning: em++: warning: linker setting ignored during compilation: 'MODULARIZE' [-Wunused-command-line-argument]

Should we be concerned or are these just relevant for emscripten < 3.1.1?

• When I started the example locally with Chrome on the MacBook, the circle showed a very slow framerate (around 10 fps) when moving the mouse, but that did not happen when I used port forwarding and ran it on Windows.

@flupke If you'd like to, add yourself to the "Notable Contributions" section in the README.md and also add the webassembly example to the "Examples" section with a link to wasm-example/README.md.

[flupke]

• In wasm-example/ make build I get the following repetitive warnings on macOS and Ubuntu:

warning: em++: warning: linker setting ignored during compilation: 'EXPORTED_RUNTIME_METHODS' [-Wunused-command-line-argument]
warning: em++: warning: linker setting ignored during compilation: 'ERROR_ON_UNDEFINED_SYMBOLS' [-Wunused-command-line-argument]
warning: em++: warning: linker setting ignored during compilation: 'MAX_WEBGL_VERSION' [-Wunused-command-line-argument]
warning: em++: warning: linker setting ignored during compilation: 'MODULARIZE' [-Wunused-command-line-argument]

Should we be concerned or are these just relevant for emscripten < 3.1.1?

Yes EMCC_FLAGS is used for both link and compilation, and I don't see more specialized env vars here: https://emscripten.org/docs/tools_reference/emcc.html#environment-variables

I don't think putting these flags in the rust build script is a good idea, since e.g. ERROR_ON_UNDEFINED_SYMBOLS is a workaround for specific emscripten versions.

So I guess we're stuck with these warnings, unfortunately.

• When I started the example locally with Chrome on the MacBook, the circle showed a very slow framerate (around 10 fps) when moving the mouse, but that did not happen when I used port forwarding and ran it on Windows.

Interesting, which browser / version were you using? (someone told me Safari doesn't support WebGL 2, maybe that's related?)

@etiennealbert could you run some tests please?

@flupke If you'd like to, add yourself to the "Notable Contributions" section in the README.md and also add the webassembly example to the "Examples" section with a link to wasm-example/README.md.

Thanks! done in c618382

[pragmatrix]

Interesting, which browser / version were you using?

Latest Chrome Version 97.0.4692.99 (Official Build) (x86_64)

[pragmatrix]

This looks ready. Thank you a lot for your contribution @flupke. I'll add web-assembly CI builds later. Merging.