iris: separate layout allocation from lengths

src/widget/layout/scroll_area.rs

@@ -97,7 +97,7 @@ mod tests {
         let mut rsc = TestRsc { ui: Ui::default() };
         let fill = rsc.ui.widgets.add_strong(Rect::new(PaintId::WHITE)).any();
         let id = fill.id();
-        let long = Some(Len::abs(1000.0));
+        let long = Some(LayoutLen::abs(1000.0));
         let tall = rsc.ui.widgets.add_strong(Sized {
             inner: fill,
             x: (axis == Axis::X).then_some(long).flatten(),