fix: apply menu item origin as an offset

[michaeloliverx]

Add origin values to the existing item rectangle instead of replacing its coordinates.

Fixes #78.

[Laupetin]

awesome :D yep that was definitely a mistake. The thing now is that what the game does is pretty weird, no? 🤔
If you specify origin twice it will keep adding to the coordinates instead of replacing the origin.
But i guess minor detail and mistake of the game devs.

Also I am wondering how this interacts with exp rect x. When optimizing menus is enabled (default), it will inline any constant rect x into item->window.rectClient.x. That will override any value that was previously set by origin? But i guess that's the same for the vanilla behaviour?

Two things i'd usually not do but i guess the vanilla game does them so OAT should as well?

[Laupetin]

Also now that i see it: Optimizing the exp rect x to be constant only writes in the rectClient and not rect as well? Also seems to be wrong to me 😬 But i guess that should maybe be fixed in another PR.

[Laupetin]

Thank you very much ❤️ Messed that up 😅 IW4x menus seem to be perfect to my eyes at least? Pretty cool!

[Laupetin]

Also now that i see it: Optimizing the exp rect x to be constant only writes in the rectClient and not rect as well? Also seems to be wrong to me 😬 But i guess that should maybe be fixed in another PR.

addressed that in #920