Implement explicit text caching in the widget state tree

examples/toast/src/main.rs

@@ -326,10 +326,13 @@ mod toast {
 
         fn layout(
             &self,
+            tree: &Tree,
             renderer: &Renderer,
             limits: &layout::Limits,
         ) -> layout::Node {
-            self.content.as_widget().layout(renderer, limits)
+            self.content
+                .as_widget()
+                .layout(&tree.children[0], renderer, limits)
         }
 
         fn tag(&self) -> widget::tree::Tag {
@@ -517,6 +520,7 @@ mod toast {
                 10.0,
                 Alignment::End,
                 self.toasts,
+                self.state,
             )
             .translate(Vector::new(position.x, position.y))
         }