Allow drawing the world on only part of the screen

[slipher]

When reading #1383 it occurred to me that another good alternative to changing the FOV for wide screens in fullscreen mode could be drawing only on part of the screen. You know, like when watching a movie and there are black bars on the sides. A similar functionality actually already exists in the cvar cg_viewsize. This cvar lets you shrink the area in which the world is drawn. It's not helpful for wide screens though, as it is a single scale factor which preserves the aspect ratio of the screen. Also RmlUi doesn't respond to cg_viewsize; we'd probably want to shrink the menu too and definitely the HUD.

[DolceTriade]

I thought the main reason people mess with FOV is to gain additional advantage to see more enemies ot the left and right?

[slipher]

If the aspect is really wide, stuff on the sides of the screen gets excessively stretched out. So we may want to do something here just to prevent it from looking bad in this manner.