Web: transferControlToOffscreen() breaks calls to canvas

[daxpedda]

I am using transferControlToOffscreen() to draw in a different thread, which unfortunately invalidates any calls to width() and height().

This was not really an issue until now because I can simply avoid calling Window::set_inner_size(). But later I discovered that switching to fullscreen and changing the scale factor will trigger a call to width() and height() by Winit I can't prevent.

I have two proposals to fix this:

• Detect the failure when calling HtmlCanvasElement::set_width() and just do nothing in that case. This is simple, but would require introducing custom wasm-bindgen imports because set_width() is not marked with #[catch]. This can't be fixed upstream because it's a breaking change.
• Introduce a new method to WindowExtWebSys to actually call the transferControlToOffscreen() for the user, storing a flag somewhere that the canvas is now disconnected.

Happy to make a PR of course.