[opus] 133 kbps stereo killer sample

[Agustín Dall'Alba]

Hello! I found a sample I can ABX successfully when encoded at
133.333 kbps. I was targetting 1 MB/min.

https://drive.google.com/drive/folders/0B8KWShoIrA1kQzR1Z0FFRUlfcEU

floex.wav is 4:54–5:04 of a lossless copy of 'Forget-me-not' by
Floex, downloaded from http://store.floex.cz/album/zorya

floex-133.opus was created with `opusenc --bitrate 133.333333 floex.wav
floex-133.opus`, opus-tools 0.1.9 (using libopus 1.1.4).

I used a Sony MDR-V6 pair of headphones and my laptop's oonboard audio. I
detected the artifact when I was listening casually from my phone with the
same headphones.

In the second to last note of the crescendo (at ~5 s in the sample) there's
a slight flutter in the original that turns more prominent and tape-like
after encoding.

I also tried encoding at 128 kbps and 150 kbps.

ABX results:
- floex-133.opus.wav 10/10, p-value<0.001
- floex-150.opus.wav 4/10, p-value<0.83
- floex-128.opus.wav 17/20, p-value<0.002

Only at 150 kbps is Opus transparent to me for this sample.

Cheers