Generic-ify the HUD's ammo count

[Doom2fan]

Right now, the ammo count is somewhat hardcoded for each gun and relies on the ammo count being an inventory item.
It'd be a good idea to handle this by asking the weapon for its ammo count and max ammo, but then how do we handle providing the bullet graphics, if it's supposed to work for any HUD type?

• It could work by ammo kinds (e.g., bullet, battery/meter)
• We could have a system in the HUD where you define "ammo bar renderers" and those can handle different kinds of ammo.