Emergency shutters now 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.
* Emergency shutters can be examined, if close enough, for a quick-summary of the nearby pressure and temperature. No gas composition.

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 icon update it would instead display the pressure warning lights.
![Too Hot!](https://www.dropbox.com/s/4b259b9j5nvblq3/EmergencyShutters.png?dl=1)
Disgusting. So glad F are not used anywhere in spessmen.