Limit number of vehicle route step markers for a single station to 10.

If more than 10, draw an overflow marker instead.
Minor refactoring.

src/lang/english.txt

@@ -4945,6 +4945,7 @@ STR_VIEWPORT_SHOW_VEHICLE_ROUTE_STEP_STATION                    :{BLACK}ST
 STR_VIEWPORT_SHOW_VEHICLE_ROUTE_STEP_WAYPOINT                   :{GRAY}WP
 STR_VIEWPORT_SHOW_VEHICLE_ROUTE_STEP_IMPLICIT                   :{GRAY}IM
 STR_VIEWPORT_SHOW_VEHICLE_ROUTE_STEP_DEPOT                      :{RED}DE
+STR_VIEWPORT_SHOW_VEHICLE_ROUTE_STEP_OVERFLOW                   :{RED}{NUM}x
 
 STR_VIEWPORT_TOWN_POP                                           :{WHITE}{TOWN} ({COMMA})
 STR_VIEWPORT_TOWN                                               :{WHITE}{TOWN}