Consume by layer, not by widget

Replaces the taking mechanism with `CursorSenses::consumes`, which
decides only whether a layer stops the input reaching the layer below.
Nothing is removed from the cursor, and senses on one layer no longer
block each other: every sensor the pointer is inside runs.

Where the cursor rests stops at the top layer under it. Something
happening to the cursor stops only at a widget that answers to it, so a
click-only child does not swallow a scroll -- which is what `main` gets
wrong, where any hovered sensor blocks the layer below.

A widget the cursor has left still hears its hover ending, but is handed
no press or scroll: that input landed somewhere else. This is a hit test
rather than a consumption rule, and without it a press beside a button
fires the button it just left.

`a_click_and_a_scroll_in_one_frame_go_to_different_widgets` goes with the
per-kind taking it tested. Of the five that remain, two fail on `main`.

tests/pointer_routing.rs

@@ -1,4 +1,4 @@
-//! Input across layers: what a widget takes, what passes through it, and
+//! Input across layers: what stops at a layer, what passes through it, and
 //! where hovering stops. These drive `run_sensors` directly, which needs no
 //! GPU and no window.
 
@@ -167,27 +167,6 @@ fn a_scroll_passes_through_every_widget_that_does_not_want_it() {
     assert_eq!(overlay_clicked.take(), []);
 }
 
-#[test]
-fn a_click_and_a_scroll_in_one_frame_go_to_different_widgets() {
-    let mut ui = Ui::new();
-    let (list, button) = (full(&mut ui), full(&mut ui));
-    let scrolled = ui.listen(&list, CursorSense::Scroll);
-    let clicked = ui.listen(&button, CursorSense::click());
-    ui.stack(vec![list.any(), button.any()]);
-
-    let mut cursor = ui.cursor((50.0, 50.0));
-    cursor.scroll_delta = (0.0, 10.0).into();
-    cursor.buttons.left = ActivationState::Start;
-    ui.run(cursor);
-
-    assert_eq!(clicked.take(), [CursorSense::click()]);
-    assert_eq!(
-        scrolled.take(),
-        [CursorSense::Scroll],
-        "taking the click must not take the scroll with it"
-    );
-}
-
 #[test]
 fn only_the_topmost_listener_takes_a_press() {
     let mut ui = Ui::new();