ad_lavc: disable decoder downmix by default

Let's see how much everyone hates this. Leaving it enabled seems
problematic, because libavcodec returns an unspecific error if it
doesn't like it.

Fixes: #6300

DOCS/man/options.rst

@@ -1829,7 +1829,7 @@ Audio
     other players) ignore this for the sake of better audio quality.
 
 ``--ad-lavc-downmix=<yes|no>``
-    Whether to request audio channel downmixing from the decoder (default: yes).
+    Whether to request audio channel downmixing from the decoder (default: no).
     Some decoders, like AC-3, AAC and DTS, can remix audio on decoding. The
     requested number of output channels is set with the ``--audio-channels`` option.
     Useful for playing surround audio on a stereo system.

audio/decode/ad_lavc.c

@@ -72,7 +72,7 @@ const struct m_sub_options ad_lavc_conf = {
     .size = sizeof(struct ad_lavc_params),
     .defaults = &(const struct ad_lavc_params){
         .ac3drc = 0,
-        .downmix = 1,
+        .downmix = 0,
         .threads = 1,
     },
 };