SceneryPager singleton must not be removed while FGScenery is still alive,
so hold a reference to it in FGScenery, saving the pager from being
deleted first.
MP aircraft are loaded by a separate OSG thread (introduced after FG2.4.0).
The OSG thread also calls the "modelLoaded" callback. However, we mustn't
allow the OSG thread to call FGNasalModelData::modelLoaded directly:
FGNasalModelData isn't thread-safe. There are obvious issues (_callCount++;),
tricky issues like calling the Nasal _parser_ or creating hashes and
modifying the global Nasal namespace. It doesn't use locks to protect
against another thread executing a Nasal context or running garbage
collection. It also executes Nasal code itself (the model's "load" hook),
which we also cannot allow in a separate thread...
This patch returns all Nasal parts of MP-aircraft loading (parsing,
module creation, execution) to the main thread, while keeping the
multi-threaded OSG part (loading of MP-aircraft model files itself).
The same issue exists with scenery models (see other commit).
To summarize with 2 words: It s*cks... ;-)
Untangle AI reinit and init methods. Some code in init hooks expects to be
called once only. Derived classes should not redirect their reinit to init,
even if this seems fine for the methods of the derived class itself. This
also triggers the init methods of all base classes, which may not expect
multiple calls to their init methods (or to "init" on "reinit").
FGFX objects must be dereferenced early enough, and sound manager must be
removed late enough - otherwise openal complains about resources being
still in use when tryin to remove buffers.
Also: do not create FGFX objects for AI/MP aircraft, when AI sound is
disabled.
Add a LOD (range animation) to each AI model if the property
/sim/rendering/static-lod/ai is set to any number greater than
zero. AI models get loaded but only displayed when closer than
the number given in the property (in meters).
If the property is absent or it's value is less or equal 0.0
than no LOD is added to the AI model.
Move the disable-HOT feature from the MP aircraft to the a/c base class,
disable HOT for all AIaircraft, since that's a fix/work-around for
#242: AI aircraft respect the user a/c only when HOT is _disabled_ for them
#221: AI aircraft don't stack at parking positions when HOT is disabled
Also generally disables HOT for ballistic and other models (suggested by vivian),
allowing it for ship/carrier models only.
