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>

epaint/src/text/fonts.rs

@@ -1,7 +1,7 @@
-use std::{collections::BTreeMap, sync::Arc};
+use std::collections::BTreeMap;
 
 use crate::{
-    mutex::Mutex,
+    mutex::{Arc, Mutex},
     text::{
         font::{Font, FontImpl},
         Galley, LayoutJob,