(svn r15541) -Revert (r15399): 'v->cargo_type' is also used in other places, which cannot accept CT_INVALID.

-Codechange: Add Engine::GetDefaultCargoType() and Engine::CanCarryCargo() and use them.
-Fix [FS#2617]: When articulated parts have no available default cargo, use the cargo type of the first part for livery selection.
-Change: To decide whether a vehicle is refittable do not test its current capacity for being zero, but always use the 'capacity property'.
  Note: The property is used unmodifed without calling CB 15/36. By setting it to a non-zero value and returning zero in the callback vehicles can be refitted to/from zero capacity for e.g. livery effects.
  Note: It is intentional that you cannot control refittability by CB 36.

src/engine_base.h

@@ -50,6 +50,8 @@ struct Engine : PoolItem<Engine, EngineID, &_Engine_pool> {
 
 	inline bool IsValid() const { return this->info.climates != 0; }
 
+	CargoID GetDefaultCargoType() const;
+	bool CanCarryCargo() const;
 	Money GetRunningCost() const;
 	Money GetCost() const;
 	uint GetDisplayMaxSpeed() const;