made painter actually how I wanted it (draw now takes in an owned painter)

src/core/rect.rs

@@ -28,7 +28,7 @@ impl Rect {
 }
 
 impl<Ctx> Widget<Ctx> for Rect {
-    fn draw(&self, painter: &mut Painter<Ctx>) {
+    fn draw(&self, mut painter: Painter<Ctx>) {
         painter.write(RectPrimitive {
             color: self.color,
             radius: self.radius,