Reduce viewport invalidation area of signal state changes

src/rail.h

@@ -456,6 +456,8 @@ static inline Money SignalMaintenanceCost(uint32 num)
 	return (_price[PR_INFRASTRUCTURE_RAIL] * 15 * num * (1 + IntSqrt(num))) >> 8; // 1 bit fraction for the multiplier and 7 bits scaling.
 }
 
+void MarkSingleSignalDirty(TileIndex tile, Trackdir td);
+
 void DrawTrainDepotSprite(int x, int y, int image, RailType railtype);
 int TicksToLeaveDepot(const Train *v);