Mantaflow Air density

Mantaflow liquids act as if the domain is in a vacuum. We need a feature where the air isn't skipped.

Currently, if you create empty space inside of a liquid the simulator acts as if it is just an empty compressable space. That removes much-needed realism from the simulation.

In my test, you can see that the air bubble just shrinks (it gets filled) instead of rising to the top as it should.