Allow widescreen fix to center HUD at 16:9

[WispOfTheWinds]

For those playing on 21:9 monitors etc. it may be nice to have the HUD centered at a 16:9 ratio. This could be enabled with chimera_widescreen_fix 3.

I haven't dug into the code at this point to see, but if there's already code to dynamically center the HUD at 4:3 I can't imagine it would be awfully difficult to add in a calculation for 16:9.