Make Iris layout dependencies explicit

core/src/widget/mod.rs

@@ -16,7 +16,7 @@ pub use view::*;
 pub use widgets::*;
 
 pub trait Widget: Any {
-    fn draw(&mut self, painter: &mut Painter) -> Size;
+    fn draw(&mut self, painter: &mut Painter);
 
     /// An exact, context-free length known without drawing or inspecting children.
     fn size_hint(&self, _axis: Axis) -> Option<Len> {
@@ -45,8 +45,8 @@ pub trait Widget: Any {
 }
 
 impl Widget for () {
-    fn draw(&mut self, _: &mut Painter) -> Size {
-        Size::ZERO
+    fn draw(&mut self, painter: &mut Painter) {
+        painter.set_size(Size::ZERO);
     }
 
     fn is_size_independent(&self) -> bool {