Door layer changes.
Created by: ComicIronic
Firedoors are now always under airlocks when open, and always above airlocks when closed. Added the base_layer var to remove magic numbers. This should make opening and closing doors easier. Fixes #4676 (closed), fixes #4669 (closed). EXAMPLE: http://puu.sh/i17T7/9eabdfdb2e.webm