Fix alpha blending in WebGL2 backend (#650)

Add a render-to-texture step with an sRGBA8 texture

egui_demo_lib/src/apps/demo/painting.rs

@@ -12,7 +12,7 @@ impl Default for Painting {
     fn default() -> Self {
         Self {
             lines: Default::default(),
-            stroke: Stroke::new(2.0, Color32::LIGHT_BLUE), // Thin strokes looks bad on web
+            stroke: Stroke::new(1.0, Color32::LIGHT_BLUE),
         }
     }
 }