fix: Use Opus in the CBR mode

VBR is susceptible to a transcription attack, where words can be
deducted from bandwidth fluctuations, even despite the audio being
encrypted. Toxcore does add padding, but it's just 0-7 bytes, to pad to
a 8 byte boundary, which might not be enough. CBR is safe from this
attack, it is the industry recommendation to use CBR: "Applications
conveying highly sensitive unstructured information SHOULD NOT use
codecs in VBR mode."[1], and is what other secure messengers use too,
e.g. Signal.

Here are some papers on this topic:
- A. M. White, A. R. Matthews, K. Z. Snow and F. Monrose, "Phonotactic
  Reconstruction of Encrypted VoIP Conversations: Hookt on Fon-iks,"
  2011 IEEE Symposium on Security and Privacy, Oakland, CA, USA, 2011,
  pp. 3-18, doi: 10.1109/SP.2011.34.
- L. A. Khan, M. S. Baig, and Amr M. Youssef. Speaker recognition
  from encrypted VoIP communications. Digit. Investig. 7, 1–2 (October,
  2010), 65–73. https://doi.org/10.1016/j.diin.2009.10.001
- C. V. Wright, L. Ballard, S. E. Coull, F. Monrose and G. M. Masson,
  "Spot Me if You Can: Uncovering Spoken Phrases in Encrypted VoIP
  Conversations," 2008 IEEE Symposium on Security and Privacy (sp 2008),
  Oakland, CA, USA, 2008, pp. 35-49, doi: 10.1109/SP.2008.21.

Thanks to an IRC user who asked to remain anonymous for sending the
diff.

[1] https://datatracker.ietf.org/doc/html/rfc6562#section-3

toxav/audio.c

@@ -377,6 +377,21 @@ static OpusEncoder *create_audio_encoder(const Logger *log, uint32_t bit_rate, u
         goto FAILURE;
     }
 
+    /*
+     * The libopus library defaults to VBR, which is unsafe in any VoIP environment
+     * (see for example doi:10.1109/SP.2011.34). Switching to CBR very slightly
+     * decreases audio quality at lower bitrates.
+     *
+     * Parameters:
+     *  `[in]`    `x`   `opus_int32`: Whether to use VBR mode, 1 (VBR) is default
+     */
+    status = opus_encoder_ctl(rc, OPUS_SET_VBR(0));
+
+    if (status != OPUS_OK) {
+        LOGGER_ERROR(log, "Error while setting encoder ctl: %s", opus_strerror(status));
+        goto FAILURE;
+    }
+
     /*
      * Configures the encoder's use of inband forward error correction.
      * Note: