eframe support for wgpu on the web (#2107)

* basic working wgpu @ webgl on websys

* fix glow compile error

* introduced WebPainter trait, provide wgpu renderstate

* WebPainterWgpu destroy implemented

* make custom3d demo work on wgpu backend

* changelog entry for wgpu support eframe wasm

* remove temporary logging hack

* stop using pollster for web
we're actually not allowed to block - this only worked because wgpu on webgl doesn't actually cause anything blocking. However, when trying webgpu this became an issue

* revert cargo update

* compile error if neither glow nor wgpu features are enabled

* code cleanup

* Error handling

* Update changelog with link

* Make sure --all-features work

* Select best framebuffer format from the available ones

* update to wasm-bindgen 0.2.83

* Fix typo

* Clean up Cargo.toml

* Log about using the wgpu painter

* fixup wgpu labels

* fix custom3d_wgpu_shader ub padding

* remove duplicated uniforms struct in wgsl shader for custom3d

* Update docs: add async/await to the web 'start' function

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/eframe/CHANGELOG.md

@@ -13,6 +13,7 @@ NOTE: [`egui-winit`](../egui-winit/CHANGELOG.md), [`egui_glium`](../egui_glium/C
 * Fix: app state is now saved when user presses Cmd-Q on Mac ([#2013](https://github.com/emilk/egui/pull/2013)).
 * Added `center` to `NativeOptions` and `monitor_size` to `WindowInfo` on desktop ([#2035](https://github.com/emilk/egui/pull/2035)).
 * Web: you can access your application from JS using `AppRunner::app_mut`. See `crates/egui_demo_app/src/lib.rs`.
+* Web: You can now use WebGL on top of `wgpu` by enabling the `wgpu` feature (and disabling `glow` via disabling default features) ([#2107](https://github.com/emilk/egui/pull/2107)).
 
 
 ## 0.19.0 - 2022-08-20