LinPorts.Rider.Input, LinBackends: motion-notify signal handler to poll actual pointer coords rather than use the ones supplied in the GdkEventMotionNotify: this way, PollCursorMsg and Rinder.Input always return actual mouse coordinates. This prevents lagging that occured when mouse handling would take a long time. Now no need to call f.Input in response to PollCursorMsg => WM_LBUTTONDOWN is never prematurely processed; for the user, no clicks are ever lost when navigating menus. In StdDocuments, Reroute TickMsg to the overlaid view, if any. Prevents caret from blinking thru a dropped menu