Add an example demonstrating how to create multiple windows that share a single graphics "context"

[mitchmindtree]

This can be really useful for things like multi-screen displays, VJ software, etc.

There's a short discussion on this here.

[mitchmindtree]

This might be best as a vk_multi_window.rs example, that shows how to use the Draw API to draw to one large Fbo the size of two or three windows combined and then draw each section of that Fbo to each of the windows.

[JoshuaBatty]

I'm happy to make a vk_multi_window.rs example.

[mitchmindtree]

This is the default behaviour as of #452!