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/demo/dancing_strings.rs

@@ -10,7 +10,7 @@ impl super::Demo for DancingStrings {
         "♫ Dancing Strings"
     }
 
-    fn show(&mut self, ctx: &CtxRef, open: &mut bool) {
+    fn show(&mut self, ctx: &Context, open: &mut bool) {
         use super::View as _;
         Window::new(self.name())
             .open(open)