The set of hovered widgets is two vectors that trade places, so an input
allocates nothing once they have grown, instead of building a fresh one
each time.
`run_sensors` no longer takes a `window_size`: `UiRenderState` already
holds the output size, and passing it back in was one more thing that
could disagree.
`tests/pointer_routing.rs` drops its own `Rsc`, event manager and layer
scaffolding for `iris::harness`, which is what it was standing in for.
Presses now arrive as a move and then a press, and a scroll after the
hover that precedes it, because that is what the harness delivers and
what a window does.
iris
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