switch to element defined span lens + better size fn

src/core/image.rs

@@ -10,8 +10,8 @@ impl Widget for Image {
         painter.texture(&self.handle);
     }
 
-    fn desired_size(&mut self, _: &mut SizeCtx) -> UiVec2 {
-        UiVec2::abs(self.handle.size())
+    fn desired_size(&mut self, _: &mut SizeCtx) -> Size {
+        Size::abs(self.handle.size())
     }
 }