Add replacement sprites for road waypoint buildings

Add action 5 sprite replacement support

docs/newgrf-additions-nml.html

@@ -208,6 +208,12 @@
 			<p>There is currently one misc GUI sprite.</p>
 		</td>
 	</tr>
+	<tr><td>ROAD_WAYPOINTS</td><td>4</td>
+		<td>
+			<b>Road waypoint graphics</b>
+			<p>This is the same order and format as the drive-through bus/truck road stop sprites.</p>
+		</td>
+	</tr>
 	</table>
 	<h3>Signal graphics using switches</h3>
 	<p>