Fix Widget::tag implementation of pure::Canvas

Using `P::State` can cause a panic if the `Canvas` has `()` as
`P::State` and replaces a stateless widget in a future `view` call.

lazy/src/pure/component.rs

@@ -70,8 +70,6 @@ where
     })
 }
 
-struct Tag<T>(T);
-
 struct Instance<'a, Message, Renderer, Event, S> {
     state: RefCell<Option<State<'a, Message, Renderer, Event, S>>>,
 }
@@ -132,6 +130,7 @@ where
     Renderer: iced_native::Renderer,
 {
     fn tag(&self) -> tree::Tag {
+        struct Tag<T>(T);
         tree::Tag::of::<Tag<S>>()
     }