seqplay.cc: Remove WIN32 defines

This code is not compatible with WIN32 at all at the moment, no need
to keep broken compatibility around.
Can be added back later in case we support other MIDI backends than ALSA.
WIP: add MIDI command queue to adlseq

Add a command queue, queue incoming ALSA midi events in a thread,
and process them in the main thread. Make sure that processing is always
some duration ahead of incoming events, this should result in much
better relative timing of events compared to
always-process-512-samples-at-a-time.
README.md update
Update SAMPLE_MULT_OUTPUT_FLOAT to 0.33