Emergency shutters down have pressure/temperature difference overlays.

* Emergency shutters now flash when sufficient pressure difference is detected between the areas they separate.
* Emergency shutters also flash a red light if the temperature is greater than 50C on any side, with the light showing the direction.
* Emergency shutters also flash a blue light if the temperature is less than 0C on any side, with the light showing the direction.

Based on the emergency shutters from Paradise, adapted to our code, with an added process-delay to reduce potential lag.
Also changed the way doors handle layer changes to allow emergency shutters to display as expected when opened/closed.

Here I have accidentally set the engineering hard storage on fire. Note the hot-warning lamp towards the right.
Next update it would instead display the pressure warning lights.
![Too Hot!](https://www.dropbox.com/s/4b259b9j5nvblq3/EmergencyShutters.png?dl=1)