(svn r3996) -Fix: Slope and height information returned for some tile types is wrong

This leads to graphical glitches when drawing foundations.
This doesn't fix all problems, but at least some of them.

dummy_land.c

@@ -16,12 +16,12 @@ static void DrawTile_Dummy(TileInfo *ti)
 
 static uint GetSlopeZ_Dummy(const TileInfo* ti)
 {
-	return GetPartialZ(ti->x & 0xF, ti->y & 0xF, ti->tileh) + ti->z;
+	return 0;
 }
 
 static uint GetSlopeTileh_Dummy(const TileInfo* ti)
 {
-	return ti->tileh;
+	return 0;
 }
 
 static int32 ClearTile_Dummy(TileIndex tile, byte flags)