Improve buffer timestamping Make buffer timestamps more accurate and, more importantly, actually representative of the MIDI events timing. Previously, buffers were only sent with timetamps aligned at a 10ms boundary which was just wrong, now the buffer timestamp represents the real time of the MIDI event. Conveniently, the ALSA sequence API supports scheduling events in the future so the sequencing infrastructure can be used to have the tick delivered at the right time, avoiding any custom scheduling mechanism. The sequencer queue is started on the first transition to PLAYING, and the delay is also calculated when the pipeline goes into PLAYING.