Add variables for constant first-person model offset

[Calinou]

This can be used to make the first-person weapon model less intrusive (or left-handed, although this isn't ideal as the model itself won't be mirrored).

Preview

Various combinations of /firstpersonside, /firstpersonup and /firstpersondist:

[SniperGoth]

Oh this could be useful, without having to manually change the offset values on the cfg files.

[Calinou]

Oh this could be useful, without having to manually change the offset values on the cfg files.

Indeed, this is also useful for people working on the models themselves 🙂

In a future PR, we can explore adding a variable for a proper left-handed mode that will flip the weapon models.

[Calinou]

Superseded by #1324 (I used the wrong fork branch here).