Think about implementing a crosshair

[wendivoid]

I would like the crosshair to be configurable by the user in the main menu as well as having a default for weapon. I image adding yet another new object type to Package named Crosshairs and weapons can specify their default in WeaponMeta. The player can override this default with an optional crosshair field in zombrr_arena::PlayerOptions.

In a previous version this was as simple as placing an image in the center of the screen. But now it needs to be placed using the weapons EyePoint & ShotPoint as the raycast source.

TODO: investigate ways to implement this