Add Shape::Callback to do custom rendering inside of an egui UI (#1351)

* Add Shape::Callback to do custom rendering inside of an egui UI
* Use Rc<glow::Context> everywhere
* Remove trait WebPainter
* Add glow::Context to epi::App::setup

epaint/src/shape_transform.rs

@@ -51,5 +51,8 @@ pub fn adjust_colors(shape: &mut Shape, adjust_color: &impl Fn(&mut Color32)) {
             adjust_color(&mut bezier.fill);
             adjust_color(&mut bezier.stroke.color);
         }
+        Shape::Callback(_) => {
+            // Can't tint user callback code
+        }
     }
 }