Merge branch 'sev/hud_ammo_panel' into 'master'

Sev/hud ammo panel

This branch makes the following additions and changes to the HUD ammo panel:

- New cvar *hud_panel_ammo_noncurrent_alpha*, to set the alpha of all ammo items not currently used (was hardcoded)
- New cvar *hud_panel_ammo_noncurrent_scale*, to scale the size of not currently used ammo items in relation to the current ammo
- Never blacks out the current ammo, even if 0. A red 0 is easier to see and understand than a black item
- The red text color for low ammo is slightly brighter, for better visibility
- In conformance to established color codes, infinite ammo text is now green instead of blue
- Infinite ammo text shows the proper infinity symbol, instead of 999

The purpose of this is to improve consistency and to allow setting appart current from noncurrent ammo,
as an alternative to using *hud_panel_ammo_onlycurrent*.

Luma WIP:
![hud_ammo](https://gitlab.com/uploads/xonotic/xonotic-data.pk3dir/119d9c32a0/hud_ammo.png)

See merge request !84