Fix Gas Being Sucked Out Into Space

[vonVile]

All gases should be sucked out into space as a locked constant speed. The same amount lost at 5K per tile should be the same when it's 5mkg per tile.

In my current game I'm playing on the cracked planet where it's taking 50+ cycles for Oxygen and Polluted Oxygen to dissipate from the underside of the top half. It should have been gone ages ago. The reason I haven't just put a Pump there is that I was expecting it all to go away fast with it being so little there would be no need, but it stays because O2 and PO2 are designed to go only up and I was very busy having Dupes manually pumping Magma into space to reach the lower half of the planet.