(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/newgrf_engine.cpp

@@ -850,12 +850,7 @@ static const SpriteGroup *GetVehicleSpriteGroup(EngineID engine, const Vehicle *
 
 	const Engine *e = GetEngine(engine);
 
-	/* Engines are not available, when their cargo is not available.
-	 * But that does not apply to articulated parts. */
-	if (cargo == CT_INVALID) cargo = CT_DEFAULT;
-
 	assert(cargo < lengthof(e->group));
-
 	group = e->group[cargo];
 	if (group != NULL) return group;