(svn r2595) -Codechange: Introduced "IsSteepTileh" to find whether a tile is steep

(i.e. spans two height levels) and use it throughout the code.
-Codechange: Add CanBuildDepotByTileh to find if a tile is suitable to
build a depot on it. Wraps some bitmagic which seems quite unreadable at
first glance

depot.h

@@ -1,6 +1,9 @@
 #ifndef DEPOT_H
 #define DEPOT_H
 
+/** @file depot.h Header files for depots (not hangars)
+  * @see depot.c */
+
 #include "pool.h"
 #include "tile.h"
 
@@ -112,6 +115,28 @@ static inline DiagDirection GetDepotDirection(TileIndex tile, TransportType type
 	}
 }
 
+/**
+	Find out if the slope of the tile is suitable to build a depot of given direction
+	@param direction The direction in which the depot's exit points. Starts with 0 as NE and goes Clockwise
+	@param tileh The slope of the tile in question
+	@return true if the construction is possible
+
+
+    This is checked by the ugly 0x4C >> direction magic, which does the following:
+      0x4C is 0100 1100 and tileh has only bits 0..3 set (steep tiles are ruled out)
+      So: for direction (only the significant bits are shown)<p>
+      00 (exit towards NE) we need either bit 2 or 3 set in tileh: 0x4C >> 0 = 1100<p>
+      01 (exit towards SE) we need either bit 1 or 2 set in tileh: 0x4C >> 1 = 0110<p>
+      02 (exit towards SW) we need either bit 0 or 1 set in tileh: 0x4C >> 2 = 0011<p>
+      03 (exit towards NW) we need either bit 0 or 4 set in tileh: 0x4C >> 3 = 1001<p>
+      So ((0x4C >> p2) & tileh) determines whether the depot can be built on the current tileh
+*/
+static inline bool CanBuildDepotByTileh(uint32 direction, uint tileh)
+{
+	return (0x4C >> direction) & tileh;
+}
+
+
 Depot *GetDepotByTile(TileIndex tile);
 void InitializeDepot(void);
 Depot *AllocateDepot(void);