Expose touchpad "hold" gesture to stop kinetic scrolling

[jondkinney]

Problem

On a touchpad the platform convention (macOS, GNOME, …) is that resting two
fingers on the trackpad stops an in-progress kinetic / momentum scroll — the
"touch to stop" behavior. egui can't react to this today: a stationary
finger-rest produces no pointer or scroll event, so a ScrollArea's kinetic
coast (its vel) — or any app-synthesized momentum — can't be halted by touch.

Enabling upstream change

winit is adding WindowEvent::HoldGesture { device_id, phase }, which surfaces
the Wayland hold gesture (zwp_pointer_gestures_v1 — whose protocol explicitly
names "cancel kinetic scrolling" as its purpose): rust-windowing/winit#4585.
Started when fingers rest, Ended on lift, Cancelled when the rest turns
into a scroll.

Once that lands (and egui bumps to a winit that has it), egui-winit is the
natural place to translate it.

Design question

egui core has no gesture/hold event today, and egui-winit already translates
platform gestures into semantic egui events rather than mirroring winit's
variants — e.g. winit PinchGesture → egui::Event::Zoom. Following that
pattern, how would you want the hold gesture exposed?

1. A semantic egui::Event (e.g. a pointer-gesture / scroll-control event)
   that widgets and apps can read.
2. Direct ScrollArea integration — egui-winit sets an internal flag that
   ScrollArea reads to zero its kinetic vel on hold, with no public event.
3. Both — a public event plus ScrollArea consuming it.

No strong opinion on the surface here; happy to implement whichever you prefer.

Platform note

Wayland-only in winit for now. macOS already stops OS-provided momentum on
touch, so it doesn't strictly need this for that path — but the event would
still let egui's own drag-kinetic vel (and other platforms) honor touch-to-stop
consistently.

Motivation / prior art

We hit this building macOS-style kinetic + rubber-band scrolling for Linux
touchpads (libinput doesn't synthesize trackpad momentum, so we synthesize it
ourselves). To get touch-to-stop we currently bridge winit's hold event through
a forked egui-winit into a private egui-memory flag our scroll code reads — it
works, but a first-class egui signal would drop the fork and let ScrollArea
itself benefit.

Scope / gating

Forward-looking: needs winit#4585 merged and egui on a winit with HoldGesture.
Filing now to settle the API direction so the egui-winit + ScrollArea wiring
can follow.