Make Overlay aware of viewport

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

widget/src/themer.rs

@@ -180,6 +180,7 @@ where
         tree: &'b mut Tree,
         layout: Layout<'_>,
         renderer: &Renderer,
+        viewport: &Rectangle,
         translation: Vector,
     ) -> Option<overlay::Element<'b, Message, Theme, Renderer>> {
         struct Overlay<'a, Message, Theme, NewTheme, Renderer> {
@@ -287,7 +288,7 @@ where
 
         self.content
             .as_widget_mut()
-            .overlay(tree, layout, renderer, translation)
+            .overlay(tree, layout, renderer, viewport, translation)
             .map(|content| Overlay {
                 to_theme: &self.to_theme,
                 content,