Put everything in Context behind the same Mutex (#1050)

* Move all interior mutability from Context to CtxRef and make it a handle
* Rename `CtxRef` to `Context`
* The old `Context` is now `ContextImpl` and is non-pub
* Add benchmark Painter::rect

Co-authored-by: Daniel Keller <dklr433@gmail.com>

egui_demo_lib/src/apps/http_app.rs

@@ -67,7 +67,7 @@ impl epi::App for HttpApp {
         "⬇ HTTP"
     }
 
-    fn update(&mut self, ctx: &egui::CtxRef, frame: &epi::Frame) {
+    fn update(&mut self, ctx: &egui::Context, frame: &epi::Frame) {
         if let Some(receiver) = &mut self.in_progress {
             // Are we there yet?
             if let Ok(result) = receiver.try_recv() {