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/src/epi.rs

@@ -455,6 +455,7 @@ pub struct WebOptions {
     /// Which version of WebGl context to select
     ///
     /// Default: [`WebGlContextOption::BestFirst`].
+    #[cfg(feature = "glow")]
     pub webgl_context_option: WebGlContextOption,
 }
 
@@ -464,6 +465,7 @@ impl Default for WebOptions {
         Self {
             follow_system_theme: true,
             default_theme: Theme::Dark,
+            #[cfg(feature = "glow")]
             webgl_context_option: WebGlContextOption::BestFirst,
         }
     }