(svn r3040) Don't deduce the cursor sprites when building rails from magic numbers, but fetch them from the central rail info array.

rail.h

@@ -171,6 +171,17 @@ typedef struct RailtypeInfo {
 		SpriteID convert_rail;       ///< button for converting rail
 	} gui_sprites;
 
+	struct {
+		CursorID rail_ns;
+		CursorID rail_swne;
+		CursorID rail_ew;
+		CursorID rail_nwse;
+		CursorID autorail;
+		CursorID depot;
+		CursorID tunnel;
+		CursorID convert;
+	} cursor;
+
 	struct {
 		StringID toolbar_caption;
 	} strings;