demux: use no overlapping packets for lossless audio

Worthless optimization, but at least it justifies that the
--audio-backward-overlap option has an "auto" choice. Tested with PCM
and FLAC.

DOCS/man/options.rst

@@ -560,7 +560,8 @@ Playback Control
 
     The solution is to feed a previous packet to the decoder each time, and then
     discard the output. This option controls how many packets to feed. The
-    ``auto`` choice is currently hardcoded to 1 for audio, and 0 for video.
+    ``auto`` choice is currently hardcoded to 0 for video, and uses 1 for lossy
+    audio, 0 for lossless audio.
 
     ``--video-backward-overlap`` can potentially handle intra-refresh video,
     depending on the exact conditions. You may have to use the

common/av_common.c

@@ -292,6 +292,13 @@ const char *mp_codec_from_av_codec_id(int codec_id)
     return name;
 }
 
+bool mp_codec_is_lossless(const char *codec)
+{
+    const AVCodecDescriptor *desc =
+        avcodec_descriptor_get(mp_codec_to_av_codec_id(codec));
+    return desc && (desc->props & AV_CODEC_PROP_LOSSLESS);
+}
+
 // kv is in the format as by OPT_KEYVALUELIST(): kv[0]=key0, kv[1]=val0, ...
 // Copy them to the dict.
 void mp_set_avdict(AVDictionary **dict, char **kv)

common/av_common.h

@@ -19,6 +19,7 @@
 #define MP_AVCOMMON_H
 
 #include <inttypes.h>
+#include <stdbool.h>
 
 #include <libavutil/avutil.h>
 #include <libavutil/rational.h>
@@ -44,6 +45,7 @@ void mp_add_lavc_encoders(struct mp_decoder_list *list);
 char **mp_get_lavf_demuxers(void);
 int mp_codec_to_av_codec_id(const char *codec);
 const char *mp_codec_from_av_codec_id(int codec_id);
+bool mp_codec_is_lossless(const char *codec);
 void mp_set_avdict(struct AVDictionary **dict, char **kv);
 void mp_avdict_print_unset(struct mp_log *log, int msgl, struct AVDictionary *d);
 int mp_set_avopts(struct mp_log *log, void *avobj, char **kv);

demux/demux.c

@@ -33,6 +33,7 @@
 #include "options/m_config.h"
 #include "options/m_option.h"
 #include "mpv_talloc.h"
+#include "common/av_common.h"
 #include "common/msg.h"
 #include "common/global.h"
 #include "common/recorder.h"
@@ -952,8 +953,8 @@ static void demux_add_sh_stream_locked(struct demux_internal *in,
     switch (ds->type) {
     case STREAM_AUDIO:
         ds->back_preroll = in->opts->audio_back_preroll;
-        if (ds->back_preroll < 0)
-            ds->back_preroll = 1; // auto
+        if (ds->back_preroll < 0) // auto
+            ds->back_preroll = mp_codec_is_lossless(sh->codec->codec) ? 0 : 1;
         break;
     case STREAM_VIDEO:
         ds->back_preroll = in->opts->video_back_preroll;