Bring cargo wagons up to par with fluid wagons

[Xemozu]

What?

Implement the missing sprites/animations for opening and closing cargo wagons compartment on curved rails. Now that fluid wagons can load/unload on curved sections with a nice animation, it seems that cargo wagons are not at the same level of polish.

Why?

Since 2.1 is the final release, this is one of those things that makes the game feel polished all around. As a new player, loading/unloading on curved rails with inserters feels like an unfinished feature due to the missing opening/closing animation.