Function mix_seq_clip

fn mix_seq_clip(
    scratch: &mut Scratch,
    project: &Project,
    clip: &Clip,
    ext: &[Option<(f64, f64, &'_ Transition, bool)>; 2],
    t0: f64,
    pool: &mut DecoderPool,
    out: &mut [f32],
    depth: usize,
)

One Sequence clip on a video track: recursively mix its sequence’s tracks at source rate into a scratch buffer, then treat that buffer exactly like clip source audio (resample + gains). Nested tracks keep their own mute/solo but not their own buses — the whole sub-mix goes to the bus of the Sequence clip that hosts it.