Make Iris layout dependencies explicit

src/widget/position/layer.rs

@@ -6,12 +6,12 @@ pub struct LayerOffset {
 }
 
 impl Widget for LayerOffset {
-    fn draw(&mut self, painter: &mut Painter) -> Size {
+    fn draw(&mut self, painter: &mut Painter) {
         for _ in 0..self.offset {
             painter.next_layer();
         }
-        let used = painter.widget(&self.inner);
+        let used = painter.widget(&self.inner).size();
         painter.place_used(&self.inner, used, UiRegion::FULL);
-        used
+        painter.set_size(used);
     }
 }