Feature: [NewGRF] Allow fixed layout up to 256 tiles per NewGRF rail station.

Allow using up to 256 tile layouts in property 0E or callback 24, which defines the layout to be saved into the map.

This was originally limited to 8, because station graphics above 8 referred to other station types but that was changed in 2007.

1) More efficient than using callback 14, as that needs to be checked every time a station tile is rendered.
2) The layout does not get changed when the station is changed (this may or may not be desirable!)

Using more than 256 layouts still requires callback 14.

src/newgrf_callbacks.h

@@ -38,7 +38,7 @@ enum CallbackID {
 	/** Determine whether a newstation should be made available to build. */
 	CBID_STATION_AVAILABILITY            = 0x13, // 8 bit callback
 
-	/** Choose a tile layout to draw, instead of the standard 0-7 range. */
+	/** Choose a tile layout to draw, instead of the standard range. */
 	CBID_STATION_DRAW_TILE_LAYOUT        = 0x14,
 
 	/**