[egui_web] Improve alpha blending + add WebGL2 painter

egui_web/src/painter.rs

@@ -0,0 +1,16 @@
+use wasm_bindgen::prelude::JsValue;
+
+pub trait Painter {
+    fn as_tex_allocator(&mut self) -> &mut dyn epi::TextureAllocator;
+
+    fn debug_info(&self) -> String;
+
+    /// id of the canvas html element containing the rendering
+    fn canvas_id(&self) -> &str;
+
+    fn upload_egui_texture(&mut self, texture: &egui::Texture);
+
+    fn clear(&mut self, lear_color: egui::Rgba);
+
+    fn paint_jobs(&mut self, jobs: egui::PaintJobs, pixels_per_point: f32) -> Result<(), JsValue>;
+}