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.

core/src/ui/active.rs

@@ -1,14 +1,18 @@
-use crate::{LayerId, MaskIdx, PrimitiveHandle, TextureHandle, UiRegion, WidgetId};
+use crate::{LayerId, MaskIdx, PrimitiveHandle, Size, TextureHandle, UiRegion, WidgetId};
 
 /// important non rendering data for retained drawing
 #[derive(Debug)]
 pub struct ActiveData {
     pub id: WidgetId,
     pub region: UiRegion,
+    /// What the widget said it used of `region`, the last time it drew.
+    pub size: Size,
     pub parent: Option<WidgetId>,
     pub textures: Vec<TextureHandle>,
     pub primitives: Vec<PrimitiveHandle>,
     pub children: Vec<WidgetId>,
+    /// The children whose size this widget read while drawing.
+    pub size_deps: Vec<WidgetId>,
     pub mask: MaskIdx,
     pub layer: LayerId,
 }