Add retained paints and shared text selection

examples/view.rs

@@ -20,7 +20,7 @@ struct Test {
 
 impl Test {
     pub fn new(rsc: &mut Rsc) -> Self {
-        let root = rect(Color::RED).add(rsc);
+        let root = rect(PaintId::RED).add(rsc);
         let cur = rsc.create_state(root, false);
         Self { root, cur }
     }
@@ -28,9 +28,9 @@ impl Test {
         let cur = &mut rsc[self.cur];
         *cur = !*cur;
         if *cur {
-            rsc[self.root].color = Color::BLUE;
+            rsc[self.root].set_paint(PaintId::BLUE);
         } else {
-            rsc[self.root].color = Color::RED;
+            rsc[self.root].set_paint(PaintId::RED);
         }
     }
 }