Beat notes and/or envelope timing issue

Nobody has claimed this yet.

### Bug Summary
Beat notes and short envelopes don't mix well. Not that this is much of a problem in actual music making, but it might hint at some timing detail that could be improved. I tried to replicate this using 1/192 notes, but they were fine.

#### Steps to reproduce
Put i.e. Kicker in the B&B editor, set volume envelope to 0 on everything except hold=0.01 or so. Disable all other envelopes and LFOs, turn volume envelope amount to max. Turn on all steps and play the beat.

#### Expected behavior
A steady beat.

#### Actual behavior
See screenshot. The sound cuts in and out in a pattern that changes when adjusting the BPM. And of course it varies with the sample rate and oversampling, as well.

#### Screenshot
 (3 instruments panned left/middle/right) that should be beating on every step of B&B editor for 8 bars).

#### Affected LMMS versions
Version 1.3.0-alpha.1.118+gab6da61e7 (win32/x86_64, Qt 5.9.8, GCC 7.3-win32 20180312). (e.g. master + PR #6000)

No source file, test, or entry point is named in the report. First reproduce the Kicker setup with beat notes, a short volume envelope, and all steps enabled; investigate the timing behavior across BPM, sample rate, and oversampling. Done means the beat remains steady instead of cutting in and out.

Written by the indexing model from the issue text.