Adds fullscreen using native web APIs to the stdweb and web-sys backends.
Due to limitations of browser APIs, requests for fullscreen can only be fulfilled during a short-lived user-triggered event. This commit does automatically handle that under the hood, but it does introduce unavoidable latency in full-screening the canvas.
* Keep track of what windows have requested redraw
Instead of using request_animation_frame and sending redraw request
events, just keep track of all windows that have asked for a redraw.
This doesn't handle dispatching the events
* Issue redraw events to windows that request it
* Cargo fmt
* Stop appending canvas to document in web platform
* Remove `tabindex` TODO in web backend
* Return `OsError` instead of panicking on web canvas creation
