Size a widget while drawing it, not in a pass of its own

`desired_width`/`desired_height`, `WidgetAxisFns`, `SizeCtx` and the size
cache are gone. A widget states what it used with `Painter::set_size`
while it draws, and `Painter::widget` hands back a `DrawResult` whose
`size()` both reads the child and records that this widget's size depends
on it. Reading nothing keeps the parent independent of what the child came
to.

`Span` is what the change is for. It takes each child's `size_hint` where
there is one, draws only the children that cannot answer, allocates the
flexible space, then places everything -- which deletes `desired_ortho`,
whose own comment said it "literally copies draw ... which makes this slow
and not cool".

Invalidation follows the dependency edges the draw recorded: a widget that
needs redrawing hands off to the highest ancestor that read its size,
instead of re-running a measurement to find out whether anything changed.

`Widget::size_dependence(axis)` says how much of its box a widget's
drawing depends on -- none of it, its own extent, or the whole box -- so
the retained path can keep a drawing and write a new box into it. Asked
per axis, because wrapped text depends on the width it is offered and not
on the height. `Internal` does not yet buy more than `External`: keeping a
drawing when only the room around it changed is a translation, which waits
for the move chain.

`tests/retained.rs` covers the second frame rather than the first, which
is where the bugs were: a placed child that was not recorded as one got
pruned as departed on the next draw.

`examples/text.rs` is new, since wrapping was the one thing here with no
way to see it on its own.

src/widget/image.rs

@@ -8,14 +8,15 @@ pub struct Image {
 impl Widget for Image {
     fn draw(&mut self, painter: &mut Painter) {
         painter.primitive(&self.handle);
+        painter.set_size(Size::abs(self.handle.size()));
     }
 
-    fn desired_width(&mut self, _: &mut SizeCtx) -> Len {
-        Len::abs(self.handle.size().x)
+    fn size_hint(&self, axis: Axis) -> Option<Len> {
+        Some(Len::abs(self.handle.size().axis(axis)))
     }
 
-    fn desired_height(&mut self, _: &mut SizeCtx) -> Len {
-        Len::abs(self.handle.size().y)
+    fn size_dependence(&self, _: Axis) -> SizeDependence {
+        SizeDependence::None
     }
 }