SF2: Make timing more accurate, also some fixes

I'm not saying sample-accurate, because it turns out, Fluidsynth has an internal buffer size and thus timing granularity of 64 frames. So 64 frames is the max. accuracy attainable for SF2. But it's better than nothing. Big thanks to David Henningsson of the Fluidsynth dev team, who very helpfully answered questions. A great guy.
In addition, there are some fixes to earlier commits here, which I ran into while working on the SF2 timing.