Handle ADSB altitude quirks

[hut8]

When constructing a fix from ADSB data, we need to take into account that (I think) we're only getting pressure altitude, so we have no idea what the MSL altitude is, and therefore we have no idea what the AGL altitude is. So:

• We need to ensure that the "on ground" part of Mode-S transmissions (or whatever) is used as the "inactive" flag - that means that the usual takeoff/landing check needs to be changed depending on if we're getting the data from ADSB or OGN
• I believe we already keep track of the "MSL offset" when taking off; when we detect a takeoff and let's say we determine that while they're on the ground, they're indicating that they're 100 feet AGL (because of GPS inaccuracy or wrong barometric setting) then we already save that offset somewhere. We should make sure that we take into account the real, accurate takeoff (which we only have for ADSB) and the pressure altitude that ADSB is sending us, combined with the known MSL height of the airport, and we can use that to figure out AGL and MSL height for a while.