Add access to bounds/content bounds from a scrollable viewport. (#2072)

* Add access to bounds/content bounds from a scrollable viewport in order to perform certain scrollable optimizations as a consumer.

* Move bounds/content_bounds after relative_offset as per feedback.

widget/src/scrollable.rs

@@ -1146,6 +1146,16 @@ impl Viewport {
 
         RelativeOffset { x, y }
     }
+
+    /// Returns the bounds of the current [`Viewport`].
+    pub fn bounds(&self) -> Rectangle {
+        self.bounds
+    }
+
+    /// Returns the content bounds of the current [`Viewport`].
+    pub fn content_bounds(&self) -> Rectangle {
+        self.content_bounds
+    }
 }
 
 impl State {