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.
* Use actual numeric IDs to differentiate Windows
This is generally important to identifying which window should
recieve which event, but is also specifically crucial for fixing
RedrawRequested on web.
* Cargo fmt
