made painter actually how I wanted it (draw now takes in an owned painter)

src/core/span.rs

@@ -6,7 +6,7 @@ pub struct Span {
 }
 
 impl<Ctx: 'static> Widget<Ctx> for Span {
-    fn draw(&self, painter: &mut Painter<Ctx>) {
+    fn draw(&self, mut painter: Painter<Ctx>) {
         let total = self.sums();
         let mut start = UIScalar::min();
         for (child, length) in &self.children {