(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

industry_cmd.c

@@ -1339,7 +1339,7 @@ static bool CheckIfIndustryTilesAreFree(TileIndex tile, const IndustryTileTable
 			} else {
 				if (ti.type == MP_WATER && ti.map5 == 0)
 					return false;
-				if (ti.tileh & 0x10)
+				if (IsSteepTileh(ti.tileh))
 					return false;
 
 				if (ti.tileh != 0) {