Update to 0.17, use official winit backend

[Skelebot]

• update this example to use egui_wgpu_backend 0.17 (from 0.16),
• change the egui_winit_platform winit backend to the official egui-winit backend.

I believe it's better to use the official backend in this example and create a different example for the egui_winit_platform crate (or deprecate the crate entirely in favor of a single implementation in the ecosystem, especially that both crates are very similar). I also believe that this example should live inside the egui_wgpu_backend repo (as a cargo example in a /examples folder). Discussion is welcome.