(merge conflict solving) Corrected some sprites that were left/right reversed

Ok, a brand new PR with updated sprites sheets to solve the merge conflict.

Based on the #3488 sheets.

>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.