A pointer down/up Listener around the FlutterMap now tracks whether a real
gesture is in progress, and didUpdateWidget's follow-recenter callbacks skip
the camera move while one is -- closing the timing window where a once-per-
second ambient GPS tick's postFrameCallback could land after a finger touched
the map but before flutter_map reported hasGesture: true, snapping the camera
back out from under an in-progress pan.
