switch to element defined span lens + better size fn

src/layout/widget.rs

@@ -1,11 +1,11 @@
-use crate::layout::{Painter, SizeCtx, StaticWidgetId, Ui, UiVec2, WidgetId, WidgetIdFn};
+use crate::layout::{Painter, SizeCtx, StaticWidgetId, Ui, Size, WidgetId, WidgetIdFn};
 
 use std::{any::Any, marker::PhantomData};
 
 pub trait Widget: Any {
     fn draw(&mut self, painter: &mut Painter);
-    fn desired_size(&mut self, _: &mut SizeCtx) -> UiVec2 {
-        UiVec2::MAX_SIZE
+    fn desired_size(&mut self, _: &mut SizeCtx) -> Size {
+        Size::default()
     }
 }