Function mix_tracks

fn mix_tracks(
    scratch: &mut Scratch,
    project: &Project,
    tracks: &[Track],
    t: f64,
    pool: &mut DecoderPool,
    dest: &mut Dest<'_>,
    depth: usize,
)

Accumulate (no zeroing, no clamping) the audio of tracks over [t, t + dest.frames() frames).