Improve textures (incl. builtin guns, ammo, etc.)

[ClobberXD]

I am terrible at texturing, and it'd be great if someone else can provide improved textures for

• Ammo
• Builtin gun(s).

[ClobberXD]

It's also preferable to establish a standard size for textures. While I prefer simple textures, 16x16 textures won't be enough to display enough details on the guns. I think 64x64 textures provide a nice balance between simplicity and sufficient canvas size.

[mrowlbio]

#30