Add retained paints and shared text selection

examples/task.rs

@@ -16,15 +16,15 @@ impl DefaultAppState for State {
         rsc: &mut DefaultRsc<Self>,
         _: Proxy<Self::Event>,
     ) -> Self {
-        let rect = rect(Color::RED).add(rsc);
+        let rect = rect(PaintId::RED).add(rsc);
         rect.task_on(CursorSense::click(), async move |mut ctx| {
             tokio::time::sleep(Duration::from_secs(1)).await;
             ctx.update(move |_, rsc| {
                 let rect = rect(rsc);
-                if rect.color == Color::RED {
-                    rect.color = Color::BLUE;
+                if rect.is_paint(&PaintId::RED) {
+                    rect.set_paint(PaintId::BLUE);
                 } else {
-                    rect.color = Color::RED;
+                    rect.set_paint(PaintId::RED);
                 }
             });
         })