surface: Avoid crash on drawing cockpit HUD for Aerial in space

We haven't considered about displaying HUD in vacuum.  Probably
real avionics engineers haven't, too.  If the air pressure is zero,
we cannot obtain height value from taking log of it.
Add sqserial_findobj() to prevent duplicate push to Squirrel VM

Previously, we have been pushing Serializable objects without concern
about whether the object is already been instantiated in the VM,
and it has been working mostly fine.  Recently we noticed this
mechanism's shortcomings as we add various scripts.  For instance,
If we evaluate "player.cs==player.cs", it returns false.
So we decided to insert a table to prevent duplicate instances for
actually the same object in the game to be pushed to Squirrel VM.

The table has Serializable IDs as keys and instances defined in
Squirrel as values.  If we were to push a Serializable object,
its ID is first searched in the table to find if an instance of
the object is already present in the VM.  If so, we can push a
reference to the already present one instead of creating new one.

We should remove deleted object entries in the table with the
WeakPtr's release event.
Fix Sceptor.nut to adapt previous commit
Prevent id leak in Squirrel VM object table with CoordSys

A new smart-pointer like, WeakPtr-derived class is introduced named
SqSerialPtr, in order to delete slot of the object table whose associated
object is actually destroyed.
When the actual object is being destroyed, that event triggers unlink()
method of SqSerialPtr, which in turn delete the slot for itself.
Add Doxygen comment for src/CoordSys.cpp file
Prevent id leak in Squirrel VM object table with Entity

It's just the same fix as commit ede6e92, but this time for Entity.
Now the object table seems to not leak.
Remove unused and commented include directives in Entity.cpp