feature: emscripten

[caiiiycuk]

This is reimplementation of #2233

--

Added new feature "emscripten", this feature can be used in combination with --target wasm32-unknown-emscripten to build wgpu for emscripten.

raw-gles.rs example now works also with emscripten, to run it:

1. install emsdk
2. build this example with cargo:

EMMAKEN_CFLAGS="-g -s ERROR_ON_UNDEFINED_SYMBOLS=0 --no-entry -s FULL_ES3=1" cargo build --example raw-gles --target wasm32-unknown-emscripten --features gles,emscripten,webgl

3. copy raw-gles.em.html into target/wasm32-unknown-emscripten/debug/examples/ and open it in browser

This PR also adds a workflow to check emscripten build. I think that raw-gles example is only needed example for emscripten integration it also can be a test, but it hard to do it automatically because need to install emcc. I think not worth to doing it.

Typical usage of wgpu and emscripten will be through external gles integration like in raw-gles.rs. Because windows management (like winit) crate not exists for emscripten, so it will be used with some native library like SDL/2+wgpu. SDL will initialize context and controls window/events and wgpu will be used as renderer library.

--

There is 2 identical comments in 2233 that I can't solve:

        #[cfg(feature = "emscripten")]
        let wsi_library = None;

        #[cfg(feature = "emscripten")]
        let wsi_kind = WindowKind::Unknown;

        #[cfg(not(feature = "emscripten"))]
        let (display, wsi_library, wsi_kind) = if let (Some(library), Some(egl)) =
            (wayland_library, egl.upcast::<egl::EGL1_5>())
        {
            log::info!("Using Wayland platform");
...
        } else {
            log::info!("Using default platform");
            let display = egl.get_display(egl::DEFAULT_DISPLAY).unwrap();
            (display, None, WindowKind::Unknown)
        };

        #[cfg(feature = "emscripten")]
        let display = egl.get_display(egl::DEFAULT_DISPLAY).unwrap();

Ideally we can remove this #[cfg because last else is what we actually need. But I can't find good way to jump into last else branch when feature emscripten is set. And egl::EGL1_5 is not defined when feature is emscripten.

raw-gles.rs in browser (tested in Chrome an FF):

[caiiiycuk]

Mark it as draft until fully test it on vaners + librusty_vangers

[caiiiycuk]

Tested, works fine

[kvark]

We'll probably need a separate section in README about all the ways this can be compiled for the Web

[caiiiycuk]

@kvark Regarding to emscripten, I think it's will be very handy for projects that use wgpu-native. Since this projects are on C++ and use Rust to render, they can't be ported to web until this PR. Because you can't merge 2 Modules into one (from emscripten, and from wasm32-unknown-unknown). But now, it's super simple: compile both with emscripten and link wgpu-native statically or dynamically and it will work.

So for emscripten, I think we can try to compile examples from wgpu-native. Or I think it's better we can create simple SDL1/2+wgpu-native project and write tutorial how to compile it. Because I think SDL1/2 is widely used and have first class support from emscripten.

[caiiiycuk]

Also I can do announcement in emscripten channels about this template (SDL+wgpu)

[kvark]

Yes, I'm totally with you on this. C-based projects would be able to target WebGL via webgpu-headers, it's a very cool feature. The example code should live in https://github.com/gfx-rs/wgpu-native/ for this.