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

eframe/examples/custom_font.rs

@@ -22,6 +22,7 @@ impl epi::App for MyApp {
         ctx: &egui::Context,
         _frame: &epi::Frame,
         _storage: Option<&dyn epi::Storage>,
+        _gl: &std::rc::Rc<epi::glow::Context>,
     ) {
         // Start with the default fonts (we will be adding to them rather than replacing them).
         let mut fonts = egui::FontDefinitions::default();