First pass on experimental change that allows zones to "Sleep"
Added new two-tile hazard door sprites, thanks to Ravensdale, and fixed a derp with the new ZAS sleeping stuff.
Removes the gas mask filtering stuff from gas mixtures.  Moved it into the relevant "life" code.
Merge pull request #3669 from SkyMarshal/airfixes

Zones can now sleep, multi-tile firedoors are sprited, and gas masks no longer have their fingers in gas mixture code...