Implement SEQ_EVENT_CONTROL14, SEQ_EVENT_NONREGPARAM, SND_EVENT_REGPARAM

Handle these events according to:
https://github.com/danstowell/Sonic-Visualiser/blob/master/plugin/api/dssi_alsa_compat.c
simplify seqplay loop
Report underruns in SDL audio callback
Remove unnecessary odd-samples check

The number of samples is count*2 (always stereo), so there is no need
for another check whether the number of samples is even.
Start audio after first samples

To avoid an underrun error at start, unpause audio only
after the first samples have been computed.
Use condition variable for waiting for audio buffer
Make it possible to disable reverb and postprocessing
Always process maximum number of samples

The maximum number of samples is only 512, this is an acceptable
granularity.
Make more efficient use of std::deque

Avoid seeking, prefer linear scans. Lowers CPU usage by a lot.