I added some things to the AI stuff to improve the AIThermal processing.
Before, all the thermals were processed in order, and the last one overwrote
the prior one. Now, only the data from the nearest thermal is kept. This
way a tile can be populated with many thermals, and (as long as they have the
same diameter) the one nearest the airplane correctly takes effect. This
will make us ready for the next step, "auto-thermaling", where FlightGear's
tile manager can cover a tile with thermals, and set the thermal strength
based on land-use type.
I moved the enumerated object_type to the base class. When an AI object is
created it now sets the _otype variable in the base class. This lets the AI
manager find out what kind of AI object it is dealing with, using the base
pointer. I also added a function isa() to the base class, so the manager can
process objects differently based on their type.
The AI manager now sends AIThermal processing to a different function, where
only the data from the nearest thermal is kept. After the manager processes
all the AI objects, then the results from the nearest thermal are applied to
wind-from-down.
