Reworks acid protection code, Works the same except now takes body_parts_covered flags into account and is overall a neater piece of code.
Items worn INSIDE armour only melt when that armour is melted, Backpacks, Belts and handheld items are all worn externally so melt regardless of armour
Also updates pixel_x and pixel_y randomisation of grey goo.
Merge pull request #6105 from RemieRichards/AcidProtectionUpdate

Acid protection update