When rendering, pick song end properly

[sowbug]

Options

• Last note. This is silly because almost all notes continue producing sound after the note-off.
• Last pattern. Often works for 4/4 EDM that was ending with fast-release sounds like kicks.
• First silence after last pattern. Imperfect if the song is ending with effects that have silence midway. Could fail to terminate if effects are feeding back, or they bounce around silence because of FP errors.
• Manually specified end. Something like a notch on the overall timeline.
• (from FL Studio) wrap. Low-priority, for loops.

[sowbug]

This is imperfect, but it seems to work. We wait for 64 consecutive samples that are under a certain amplitude threshold.