Support for fullscreen windowing

[jeremymeadows]

It doesn't appear that egui has support for fullscreen windowing. You can remove window decorations and set the size to fill the whole screen, but that isn't a great solution.
I added a fullscreen field to NativeOptions which gets passed through to glutin at compile time, but that isn't as useful as being able to change the mode at runtime. If anybody knows a way to get a reference to the glutin::window::Window from the egui context, I'd be happy to try to implement it and make a PR.

[emilk]

You can make a PR modeled after this one: #660

[emilk]

Closed by #1782