Return the size from draw, and fold placing back into drawing

Review response.

`Painter::set_size` is gone: `Widget::draw` returns the `Size` instead, so
a widget that does not say what it used cannot compile rather than
panicking at the widget that forgot. That also settles setting it twice --
a branch that learns something late just returns a different value.

`Painter::place` and `UiRenderState::place` are gone too. `draw_inner`
already tried to reuse an active widget's drawing before redrawing it, so
`place` was `widget_within` with its own bookkeeping bolted on; drawing a
child a second time now *is* how a parent puts it where it belongs, and a
child is deduplicated in `children` because listing one twice would move
it twice. The unification also drops `place`'s use of `ActiveData::layer`,
which is the layer a widget's own `child_layer()` left the painter on
rather than the layer it was drawn into.

What `place` did unconditionally and `widget_within` did not is record the
size dependency, so `Painter::size_hint` now records one: reading a
child's length to lay out around it is reading its size, whether it came
from a draw or from a hint. `tests/retained.rs` has a parent that only
ever reads the hint, which is the case no existing widget exercises.

`()` sizes itself `Size::default()` -- rest -- rather than zero, so it is
a gap that takes an even share of a span; `WidgetPtr` with nothing in it
does the same, since it is the same situation. `Widget::on_resize`'s
default body said `Translate` while the enum's `#[default]` said `Redraw`;
it now defers to the enum.

`was` is `old` throughout, the `OnResize` variant comments are gone, and
so are two empty `impl` blocks.

Checked: fmt, clippy and 27 tests across the workspace; `tabs` on each of
its five tabs, and `tabs` with a replay that adds two images, all
byte-identical to `upstream/main`; `view` and `minimal` likewise; and the
`text` example rendered at 1920x1200 and 900x1200 to see the paragraph
reflow and its container follow.

src/widget/position/max_size.rs

@@ -6,16 +6,14 @@ pub struct MaxSize {
     pub y: Option<Len>,
 }
 
-impl MaxSize {}
-
 impl Widget for MaxSize {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         let child = painter.widget(&self.inner).size();
         let output = painter.output_size();
-        painter.set_size(Size {
+        Size {
             x: capped(child.x, self.x, output.x),
             y: capped(child.y, self.y, output.y),
-        });
+        }
     }
 }