raw-window-handle for interoperability

[bananaturtlesandwich]

I think that it would be really nice if the Eventhandler struct implemented the raw-window-handle traits for window and display handles. Other crates which provide windows like winit, sdl, glfw and baseview do this and it means that miniquad can be used as a backend for many more crates such as wgpu. I had an attempt at this about a week ago but I wasn't experienced enough with the apple and android apis to implement these methods for those platforms.