player: use virtual time for --audio-file with ordered chapters

Apparently users prefer this behavior.

It was used for subtitles too, so move the code to calculate the video
offset into a separate function. Seeking also needs to be fixed.

Fixes #1018.

player/audio.c

@@ -255,7 +255,8 @@ double written_audio_pts(struct MPContext *mpctx)
     // to get the length in original units without speedup or slowdown
     a_pts -= buffered_output * mpctx->opts->playback_speed;
 
-    return a_pts + mpctx->video_offset;
+    return a_pts +
+        get_track_video_offset(mpctx, mpctx->current_track[0][STREAM_AUDIO]);
 }
 
 // Return pts value corresponding to currently playing audio.

player/core.h

@@ -421,6 +421,7 @@ void mp_print_version(struct mp_log *log, int always);
 // misc.c
 double get_start_time(struct MPContext *mpctx);
 double get_main_demux_pts(struct MPContext *mpctx);
+double get_track_video_offset(struct MPContext *mpctx, struct track *track);
 double rel_time_to_abs(struct MPContext *mpctx, struct m_rel_time t);
 double get_play_end_pts(struct MPContext *mpctx);
 double get_relative_time(struct MPContext *mpctx);

player/misc.c

@@ -113,6 +113,16 @@ double get_start_time(struct MPContext *mpctx)
     return mpctx->demuxer ? mpctx->demuxer->start_time : 0;
 }
 
+// Get the offset from the given track to the video.
+double get_track_video_offset(struct MPContext *mpctx, struct track *track)
+{
+    if (track && track->under_timeline)
+        return mpctx->video_offset;
+    if (track && track->is_external)
+        return get_start_time(mpctx);
+    return 0;
+}
+
 float mp_get_cache_percent(struct MPContext *mpctx)
 {
     if (mpctx->demuxer) {

player/playloop.c

@@ -236,10 +236,11 @@ static int mp_seek(MPContext *mpctx, struct seek_params seek,
         if (track->selected && track->is_external && track->demuxer) {
             double main_new_pos;
             if (seek.type == MPSEEK_ABSOLUTE) {
-                main_new_pos = seek.amount - mpctx->video_offset;
+                main_new_pos = seek.amount;
             } else {
                 main_new_pos = get_main_demux_pts(mpctx);
             }
+            main_new_pos -= get_track_video_offset(mpctx, track);
             demux_seek(track->demuxer, main_new_pos, SEEK_ABSOLUTE);
         }
     }

player/sub.c

@@ -106,14 +106,7 @@ static void update_subtitle(struct MPContext *mpctx, int order)
 
     struct osd_sub_state state;
     osd_get_sub(mpctx->osd, obj, &state);
-
-    state.video_offset = 0;
-    if (track->under_timeline) {
-        state.video_offset += mpctx->video_offset;
-    } else if (track->is_external) {
-        state.video_offset += get_start_time(mpctx);
-    };
-
+    state.video_offset = get_track_video_offset(mpctx, track);
     osd_set_sub(mpctx->osd, obj, &state);
 
     double refpts_s = mpctx->playback_pts - state.video_offset;