audio: prefer libavcodec over libmpg123

libavcodec/libavformat now handles gapless audio better. In theory, this
could be implemented with ad_mpg123 too, but since libavformat strips
metadata from mp3 files and passes pure mp3 packets to the decoders
only, this can't work by itself. Instead, the player must pass this
metadata separately. libav* do this relatively transparently over packet
"side data" (attached to AVPacket).

It might also be possible to let libmpg123 handles all this by
implementing it as demuxer that outputs PCM, but that would have other
problems, and I think it's better to make libavformat work correctly.

libmpg123 can still be used with '--ad=mpg123:mp3'.

Also see issue #1101.

audio/decode/dec_audio.c

@@ -48,10 +48,10 @@ extern const struct ad_functions ad_lavc;
 extern const struct ad_functions ad_spdif;
 
 static const struct ad_functions * const ad_drivers[] = {
+    &ad_lavc,
 #if HAVE_MPG123
     &ad_mpg123,
 #endif
-    &ad_lavc,
     &ad_spdif,
     NULL
 };