Previous fix f2ac70e inherited instrumentId/program/name
from parent MAIN track. This causes SECTION-TAB to auto-load
program 0 of whatever SoundFont is loaded on MAIN, even
though user didn't select any instrument for the section.
Section tracks now keep null instrument + synth_engine
undefined. User can still load instrument manually via
Synth button (routed by updateActiveTracks fix).
3dtours
18e177fa2d