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

egui_demo_lib/src/wrap_app.rs

@@ -47,6 +47,7 @@ impl epi::App for WrapApp {
         _ctx: &egui::Context,
         _frame: &epi::Frame,
         _storage: Option<&dyn epi::Storage>,
+        _gl: &std::rc::Rc<epi::glow::Context>,
     ) {
         #[cfg(feature = "persistence")]
         if let Some(storage) = _storage {