Make Overlay aware of viewport

... and unify `Image::float` and `Image::translate`

widget/src/stack.rs

@@ -354,6 +354,7 @@ where
         tree: &'b mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
+        viewport: &Rectangle,
         translation: Vector,
     ) -> Option<overlay::Element<'b, Message, Theme, Renderer>> {
         overlay::from_children(
@@ -361,6 +362,7 @@ where
             tree,
             layout,
             renderer,
+            viewport,
             translation,
         )
     }