separate winit and wgpu backends

[Gonkalbell]

It would be nice to separate the windowing and graphics backends, just like how imgui provide separates its backends. This will allow people to mix and match different backends.

[hasenbanck]

Hi. Have a look at these both crates that haven't been published yet:

https://github.com/hasenbanck/egui_wgpu_backend

https://github.com/hasenbanck/egui_winit_platform

Are these what you have in mind? I couldn't work wit the current backends, since they inversed the control over my app structure, so I had to write them on my own.

[Gonkalbell]

Looks good, overall. The example looks a bit more verbose because you didn't use egui::app::Backend trait & scaffolding around it, but I don't think there's a easy way to use that trait and keep the windowing platform and graphics api separate. And like you said, it inverses control of the app structure, so I think I like this divide better.

Also, looks like you already updated it to support egui = 0.2. I still haven't gotten around to that yet.

[hasenbanck]

Yeah, the inversion of control was the biggest pain point, which prevented me from using it in my current project. There is of course more scaffolding, but it's a tradeoff. Most likely there is also room for traditional backend like the web or glium backend with wgpu and winit.