This untangles some of the gigantic kitchen sink headers
in an attempt to split things into smaller files.
Also don't include gl.h, glext.h, windows.h,
and other such nonsense *everywhere*.
Lots of cleanups on the way too. More dead/unused code removal.
Remove incrFlag(), decrFlag() Lua functions.
The pool adjusts to the amount of required threads.
The implementation is as simple as possible, but should be enough
for future extensions.
This brings down decoder thread creation/destruction even more.
Also fix OpenALSystem::release() to close channels properly on shutdown,
otherwise it would deadlock with the pool, because it waits until all
threads have died off, which did not necessarily happen.
