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/egui-wgpu/src/lib.rs

@@ -17,5 +17,28 @@ pub use renderer::Renderer;
 #[cfg(feature = "winit")]
 pub mod winit;
 
-#[cfg(feature = "winit")]
-pub use crate::winit::RenderState;
+use egui::mutex::RwLock;
+use std::sync::Arc;
+
+/// Access to the render state for egui, which can be useful in combination with
+/// [`egui::PaintCallback`]s for custom rendering using WGPU.
+#[derive(Clone)]
+pub struct RenderState {
+    pub device: Arc<wgpu::Device>,
+    pub queue: Arc<wgpu::Queue>,
+    pub target_format: wgpu::TextureFormat,
+    pub renderer: Arc<RwLock<Renderer>>,
+}
+
+/// Find the framebuffer format that egui prefers
+pub fn preferred_framebuffer_format(formats: &[wgpu::TextureFormat]) -> wgpu::TextureFormat {
+    for &format in formats {
+        if matches!(
+            format,
+            wgpu::TextureFormat::Rgba8Unorm | wgpu::TextureFormat::Bgra8Unorm
+        ) {
+            return format;
+        }
+    }
+    formats[0] // take the first
+}