Add GRF feature: New Landscape

Custom graphics using actions 1/2/3.

Currently only for rock tiles

docs/newgrf-additions.html

@@ -29,6 +29,7 @@
 	<h3 id="separate_features">Features with separate pages</h3>
 	<ul>
 	<li><a href="newgrf-roadstops.html">Road stops</a></li>
+	<li><a href="newgrf-newlandscape.html">New landscape (custom graphics)</a></li>
 	</ul>
 
 	<br />
@@ -740,6 +741,7 @@
 	<table>
 	<tr><th>Feature name</th><th>Description</th></tr>
 	<tr><td><font face="monospace">road_stops</font></td><td><a href="newgrf-roadstops.html">Custom road stops (bus stops, lorry stops and road waypoints)</a></d></tr>
+	<tr><td><font face="monospace">new_landscape</font></td><td><a href="newgrf-newlandscape.html">Custom landscape graphics (currently rocky tiles only)</a></d></tr>
 	</table>
 </body>
 </html>