Add support for road stop animation, availability callback

Add animation, callback mask, general flags properties
Add animation frame variables

src/newgrf_callbacks.h

@@ -307,6 +307,15 @@ enum StationCallbackMask {
 	CBM_STATION_SLOPE_CHECK          = 4, ///< Check slope of new station tiles
 };
 
+/**
+ * Callback masks for road stops.
+ */
+enum RoadStopCallbackMask {
+	CBM_ROAD_STOP_AVAIL                = 0, ///< Availability of road stop in construction window
+	CBM_ROAD_STOP_ANIMATION_NEXT_FRAME = 1, ///< Use a custom next frame callback
+	CBM_ROAD_STOP_ANIMATION_SPEED      = 2, ///< Customize the animation speed of the road stop
+};
+
 /**
  * Callback masks for houses.
  */