slider: Use the sprite of the touch event, not the pointer

slider: Use the sprite of the touch event, not the pointer

startDragging() starts with:

  const sprite = backend.get_sprite(global.stage, event);
  this._sprite = sprite;

but then vfunc_touch_event() would only compare this._sprite to:

  sprite = backend.get_pointer_sprite(global.stage);

which never matched. And so _endDragging() was never called and
the grab was never dismissed, effectively freezing the shell.

Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8660
Fixes: c206ec693 ("slider: Use Clutter.Sprite for pointing focus accounting")
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3887>