Switch to pull-based audio

Pull audio data and render on demand instead of pushing it
to a queue.

Reduce latency as well as simplify code by getting rid of the
intermediate audio queue.

This disables adltester for now as input provides extra
challenges when the audio rendering is done in a thread.
Properly close JACK on shutdown
Avoid looping for part of a second after play

Don't emit audio after quit flag was set.
Enable fullpan by default

Note that the default OPL3 emulator doesn't support it yet, so
effectively it is still disabled by default unless you provide
something like `-emu=vintage`.
Switch post-reset messages to PrintLn

InitMessage cannot be used after StartGrid()
JACK MIDI input
alsaseq cleanup unused variable
Sanify MIDI event handling

Store MIDI events in consecutive buffers in memory.
No longer take byte and rest of message separately.
Mention JACK MIDI in README.md