remove context generic

src/core/rect.rs

@@ -27,8 +27,8 @@ impl Rect {
     }
 }
 
-impl<Ctx> Widget<Ctx> for Rect {
-    fn draw(&self, painter: &mut Painter<Ctx>) {
+impl Widget for Rect {
+    fn draw(&self, painter: &mut Painter) {
         painter.write(RectPrimitive {
             color: self.color,
             radius: self.radius,