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>

egui_demo_app/src/main.rs

@@ -9,6 +9,10 @@ fn main() {
 
     let options = eframe::NativeOptions {
         drag_and_drop_support: true,
+
+        #[cfg(feature = "wgpu")]
+        renderer: eframe::Renderer::Wgpu,
+
         ..Default::default()
     };
     eframe::run_native(