remove context generic

src/core/stack.rs

@@ -4,8 +4,8 @@ pub struct Stack {
     pub children: Vec<WidgetId>,
 }
 
-impl<Ctx> Widget<Ctx> for Stack {
-    fn draw(&self, painter: &mut Painter<Ctx>) {
+impl Widget for Stack {
+    fn draw(&self, painter: &mut Painter) {
         for child in &self.children {
             painter.draw(child);
         }