Fairly straightfowrad, programs were randomly seg faulting on exit. This
was happening due to GLUT registering a cleanup function via atexit(),
which then gets called when exit() is called.
The issue happens when exit() is called on a thread other than the GLUT
thread, which leads to the exit() call then attempting to cleanup GLUT
while the other thread is still using it, which randomly leads to seg
faults.
Fixing this is slightly annoying. We cannot stop the GLUT thread, as
the basic GLUT API (which is used on Mac OS) simply does not offer a way
to exit the glutMainLoop() call. Thus the simplest solution is to simply
make sure we call exit() on the GLUT thread, which we can fairly easily
due via the message queue.
That being the case, a new libqb_exit() API was added, which simply
regsiters the GLUT exit message and then waits for the program to end.
The atexit() handler then runs on the GLUT thread and everything works
out fine.
In the future we probably should redo the exit logic a bit so that all
the threads are actually stopped/joined to ensure the exit process is
consistent, however this is good enough for now. Also, there's plenty of
error states which call exit() which I did not address.
This fixes all the code so that all the calls to glut functions
happen on the same thread that is running GLUT.
We achieve this by creating a queue of GLUT commands to execute.
Commands can be added to the queue anywhere in the code, and then the
queue is processed on the GLUT thread via it's idle func or timer func.
The command is run and if necessary the result is provided in the
message queue object. Each object contains a completion which can be
waited on to block until the GLUT thread has processed the command.
Fixes: #66
