Avoid crashes caused by worker threads accessing the buffer manager
before it is initialized. Therefore initialize it from within the
Mixer constructor which has the side effect that it gets initialized
in console-only rendering mode as well.
Well, this commit got a bit out of hand, what with 26 files changed. Oh well.
Basically, we're using the buffermanager to dispense temporary buffers for playhandles and audioports to use.
This allows us to change the way playhandles work. Earlier, playhandles of the same track were waiting in line
to push their output to the audioport. This was of course inefficient, so now they just register themselves to the port,
then the port handles mixing the buffers.
Caveat: this is still a work in progress, the vol/pan knobs on instruments are temporarily non-functional - will be fixed in
the next commit, but I have to get some sleep now.