Build 63: armor, variable enemy density

What’s new since build 61:

• New player rule: “Bonus_Incoming_Damage_Multiplier”: allows incoming damage reduction (or increase :) + a new matching “Armor” upgrade & modifier
• New enemy rule: “Density” (physics property), defaults to 1, allows making enemies “heavier” so they can push lighter enemies around
• Added dark translucent letterbox around layouts
• Drop objects are now always at the bottom of their layer