Not sure how it worked before, but some of the old code was put in the
wrong part of the loop....
After troubleshooting what appeared to be a terrible regression
relating to concurrency (audio artifacts when running multiple overlapping
items on multiple different tracks)
, it turned out that it only happens when compiled with debug symbols,
meaning this is GCC's fault.

FFS, GCC4.8 has been a turd since it's inception, I sincerely hope they
fix it soon...

(this commit is some apparently unneeded concurrency improvements)