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/web/web_painter.rs

@@ -0,0 +1,30 @@
+use egui::Rgba;
+use wasm_bindgen::JsValue;
+
+/// Renderer for a browser canvas.
+/// As of writing we're not allowing to decide on the painter at runtime,
+/// therefore this trait is merely there for specifying and documenting the interface.
+pub(crate) trait WebPainter {
+    // Create a new web painter targeting a given canvas.
+    // fn new(canvas_id: &str, options: &WebOptions) -> Result<Self, String>
+    // where
+    //     Self: Sized;
+
+    /// Id of the canvas in use.
+    fn canvas_id(&self) -> &str;
+
+    /// Maximum size of a texture in one direction.
+    fn max_texture_side(&self) -> usize;
+
+    /// Update all internal textures and paint gui.
+    fn paint_and_update_textures(
+        &mut self,
+        clear_color: Rgba,
+        clipped_primitives: &[egui::ClippedPrimitive],
+        pixels_per_point: f32,
+        textures_delta: &egui::TexturesDelta,
+    ) -> Result<(), JsValue>;
+
+    /// Destroy all resources.
+    fn destroy(&mut self);
+}