Corrected some sprites that were left/right reversed

Equipping these items in the  right hand would makes them appear in the mob left  hand (and vice versa)

![swapped_items](https://cloud.githubusercontent.com/assets/7117411/2751651/cedc937e-c8df-11e3-8009-4a7113d7525e.PNG)

Sprites affected : (_items_lefthand.dmi_ and _items_righthand.dmi_)
* _giftbag_ (a.k.a _santa_bag_)
* _bomb_ (seems unused ?!?)
* _sombrero_
* _green sombrero_
* _shamebrero_
* _engineering jetpack_

Pretty trivial, but done nonetheless.