Add flyingLayer to units

[MEEPofFaith]

Can be set similarly to groundLayer.
If no value is given, then it get initialized to Layer.flyingUnitLow or Layer.flyingUnit depending on lowAltitude.

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[MEEPofFaith]

Question: Should flying units have units with a larger hitSize draw over other flying units with a smaller hitSize like ground units?
It's not always consistent because high altitude units inherently draw on a higher layer.