Fix a bug where bus_count was not being calculated or applied...
This had the side effect of creating tearing from improperly synchronized
buffers when more than one worker thread was used, which sounded like
drop-outs that I thought were a result of poor multi-core scaling.

As is turns out, multi-core scaling of the engine seems pretty
fucking great in the limited testing I've done since fixing it.

(somehow, the fix managed to trigger a GCC bug that caused lockups, so
also move the offending section of code to a sub-function and
give it the optimize("-O0") treatment...)