Add egui_wgpu crate (#1564)

Based on https://github.com/hasenbanck/egui_wgpu_backend

`egui-wgpu` is now an official backend for `eframe` (opt-in).

Use the `wgpu` feature flag on `eframe` and the `NativeOptions::renderer` settings to pick it.

Co-authored-by: Nils Hasenbanck <nils@hasenbanck.de>
Co-authored-by: Sven Niederberger <niederberger@embotech.com>
Co-authored-by: Sven Niederberger <73159570+s-nie@users.noreply.github.com>

examples/custom_3d_three-d/src/main.rs

@@ -6,6 +6,7 @@ fn main() {
     let options = eframe::NativeOptions {
         initial_window_size: Some(egui::vec2(550.0, 610.0)),
         multisampling: 8,
+        renderer: eframe::Renderer::Glow,
         ..Default::default()
     };
     eframe::run_native(