Codechange: Split CT_INVALID into CT_INVALID and INVALID_CARGO.

INVALID_CARGO is a CargoID and should be used for most purposes in game.
CT_INVALID is a CargoType used for defining default properties.

src/vehicle_gui_base.h

@@ -75,7 +75,7 @@ struct BaseVehicleListWindow : public Window {
 
 	/** Special cargo filter criteria */
 	enum CargoFilterSpecialType {
-		CF_NONE = CT_INVALID,       ///< Show only vehicles which do not carry cargo (e.g. train engines)
+		CF_NONE = INVALID_CARGO,    ///< Show only vehicles which do not carry cargo (e.g. train engines)
 		CF_ANY = CT_NO_REFIT,       ///< Show all vehicles independent of carried cargo (i.e. no filtering)
 		CF_FREIGHT = CT_AUTO_REFIT, ///< Show only vehicles which carry any freight (non-passenger) cargo
 	};