Add the windows-static-egl Cargo feature

[SimonSapin]

Fixes #997: this allows using Glutin together with https://crates.io/crates/mozangle

[SimonSapin]

Should mozangle be mentioned in the doc as a possible way to use this feature?

Should mozangle be added as in dev-dependencies and the program below added under examples/?

#[cfg(not(windows))]
fn main() {
    println!("This example only runs on Windows.");
}

#[cfg(windows)] extern crate mozangle;
#[cfg(windows)] extern crate glutin;

#[cfg(windows)]
fn main() {
    use glutin::GlContext;
    use glutin::os::GlContextExt;
    use glutin::os::windows::RawHandle;
    use mozangle::egl::get_proc_address;
    use mozangle::gles::ffi as gl;

    let mut events_loop = glutin::EventsLoop::new();
    let window = glutin::WindowBuilder::new();
    let context = glutin::ContextBuilder::new()
        // Requesting OpenGlEs is required to make glutin try to use EGL.
        .with_gl(glutin::GlRequest::Specific(glutin::Api::OpenGlEs, (2, 0)));
    let gl_window = glutin::GlWindow::new(window, context, &events_loop).unwrap();

    let handle = unsafe {
        gl_window.context().raw_handle()
    };
    match handle {
        RawHandle::Egl(_) => {},
        RawHandle::Wgl(_) => panic!("Expected an EGL context, found a WGL one.")
    }

    assert_ne!(gl_window.get_proc_address("glClearColor"), 0 as _);
    assert_eq!(gl_window.get_proc_address("glClearColor"), get_proc_address("glClearColor") as _);

    unsafe {
        gl_window.make_current().unwrap();
        gl::ClearColor(0., 0.5, 0., 1.);
    }

    events_loop.run_forever(|event| {
        match event {
            glutin::Event::WindowEvent { event, .. } => match event {
                glutin::WindowEvent::Closed => return glutin::ControlFlow::Break,
                glutin::WindowEvent::Resized(w, h) => gl_window.resize(w, h),
                _ => (),
            },
            _ => ()
        }

        unsafe {
            gl::Clear(gl::COLOR_BUFFER_BIT);
        }

        gl_window.swap_buffers().unwrap();
        glutin::ControlFlow::Continue
    });
}

[SimonSapin]

The example program in the previous comment is subject to rust-windowing/winit#415. (Maybe Clear + swap_buffers is slow in debug mode? And re-rendering on non-resize events is unnecessary.)

[tomaka]

I'm not fan of this feature but I guess I will merge anyway.

Please solve the conflict, and also please remove the example because for such a niche feature I don't think an example really makes sense.

[SimonSapin]

I’m not really a fan of it either. I think it’s preferable for WebRender/Wrench to use winit + EGL instead of this: servo/webrender#2529