Hand a redrawn widget the mask it inherited, not its own

`ActiveData::mask` is the mask a widget's drawing is clipped to, which is
either one it set itself or the one it inherited. `redraw` passed it back as
the *inherited* mask, so a `Masked` widget settled on its own was handed its
own mask and `set_mask` asserted -- a panic on any local redraw of one, for
as long as there has been a local-redraw path. The two are separate facts, so
`parent_mask` keeps the second.

That also states the question `remap_subtree` was asking. It compared a
widget's mask with the one threaded down from its parent to find out whether
the widget owned it; the comparison is now between the two fields on the
widget, which is the same question asked where the answer lives, and the
parameter goes.

Checked: fmt, clippy, 87 suite tests including the new one, which panics
without this; 18 core unit tests; the release oracle at 100 seeds; the
fifteen shrinker cases at 400 seeds of depth 5; and `tabs` renders
byte-identical at 1920x1200.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/cases/retained.rs

@@ -613,3 +613,18 @@ fn a_stacks_sizing_child_is_drawn_once_where_it_belongs() {
     assert_ne!(layer(front.id()), layer(background.id()));
     assert_eq!(draws.get(), 1);
 }
+
+/// A widget's own mask is not the one it inherited, and a redraw of it
+/// inherits the second: handing back the first is handing it its own mask to
+/// set a second time, which `set_mask` asserts against.
+#[test]
+fn a_masked_widget_redrawn_on_its_own_sets_its_mask_again() {
+    let mut h = Harness::new((400, 200));
+    let inner = rect(Color::BLUE).add(&mut h.rsc);
+    let masked = inner.masked().add(&mut h.rsc);
+    let other = rect(Color::RED).width(100).add(&mut h.rsc);
+    h.set_root((other, masked).span(Dir::RIGHT));
+    h.rsc.widgets_mut().get_dyn_mut(masked.id());
+    h.frame();
+    assert_corners!(h, inner, (100, 0), (400, 200));
+}