Function mix_audio_clip

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

One audio clip: read ceil(n*speed)+1 source frames at the clip’s source time, linearly resample into out (n frames) and add with per-channel gains lerped from the block start to the block end.