A pinch_data is the event<-detail
of the pinch_begin, pinch_update and pinch_end events, which are
generated for the zoom gesture of a trackpad: moving two fingers apart
or together. These events are mouse events (event->is_a
and event->is_a)
of
@mouse_device at the position of the mouse. As the event
delegates to its detail, the scale is normally requested from the event:
get(Ev, scale, Scale)
The recogniser of library(pan_zoom) zooms a figure on a
pinch.