If a GtkGestureSingle is set as touch-only, pointer events would be
discarded without giving an opportunity to the regular GtkGesture
handler to manage those.

Because the pointer events weren't actually managed by the gesture,
gtk_gesture_get_sequence_state() (rather unhelpfully here) will resort
to returning GTK_EVENT_SEQUENCE_NONE, which is in turn interpreted
by _gtk_widget_consumes_motion() as "may be handling the events for
this sequence", because gestures in this state presumably handle
the events, just that it's not "claimed" yet.

Instead, use gtk_gesture_handles_sequence(), which will perform the
expected check on the event sequence being managed, as we expect
here.