gesturemultipress: Don’t fire ::released after ::cancel

gesturemultipress: Don’t fire ::released after ::cancel

Causing a grab in the handler for ::pressed by, e.g., popping up a
context menu will cause the gesture to be canceled and, subsequently,
::end and ::released to be fired, all while the button is still
physically pressed. That results in no event being available to the
::released handler and garbage coordinates, given that
gtk_gesture_get_point() returns FALSE.

Emitting ::released can be avoided by checking the return value
gtk_gesture_get_point().