Add a waypoint ground draw flag to NewGRF road stops

Adjust overlay draw conditions

docs/newgrf-roadstops.html

@@ -91,6 +91,7 @@
 	<tr><th>Bit</th><th>Value</th><th>Meaning</th></tr>
 	<tr><td>0</td><td>1</td><td>Bay stops: Draw road type ground sprite</td></tr>
 	<tr><td>1</td><td>2</td><td>Drive through stops: Draw road/tram type overlays</td></tr>
+	<tr><td>4</td><td>4</td><td>Road waypoints: Draw sprite layout ground sprite on top of the underlying road (by default the sprite layout ground sprite is not used)</td></tr>
 	</table>
 	The default value is 3 (bits 0 and 1 both set).
 	</p>