When emulating touch, the simulating_pointer_from_touch field is
set to true. It's switched back to false when a touch_up event is
received. However we need to ensure we always send a wl_pointer.frame
event following a group of other wl_pointer events.
Since a touch_frame event is always guaranteed to come after a group
of touch events, unset simulating_pointer_from_touch in the touch_frame
handler instead of the touch_up handler. Add a new field to know whether
the touch_frame handler should stop emulation.
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