loadfile: strongly prefer that all streams are from the same program

In raw OTA MPEGTS, different programs can be entirely different virtual channels.
In web streams, different programs can be different variant bitrates,
so using streams from different programs can waste large amounts of bandwidth.

player/loadfile.c

@@ -459,6 +459,7 @@ static int match_lang(char **langs, char *lang)
  * 0b) track is not from --external-file
  * 1) track is external (no_default cancels this)
  * 1b) track was passed explicitly (is not an auto-loaded subtitle)
+ * 1c) track matches the program ID of the video
  * 2) earlier match in lang list
  * 3a) track is marked forced and we're preferring forced tracks
  * 3b) track is marked non-forced and we're preferring non-forced tracks
@@ -472,7 +473,7 @@ static int match_lang(char **langs, char *lang)
  */
 // Return whether t1 is preferred over t2
 static bool compare_track(struct track *t1, struct track *t2, char **langs,
-                          int prefer_forced, struct MPOpts *opts)
+                          int prefer_forced, struct MPOpts *opts, int preferred_program)
 {
     if (!opts->autoload_files && t1->is_external != t2->is_external)
         return !t1->is_external;
@@ -486,6 +487,11 @@ static bool compare_track(struct track *t1, struct track *t2, char **langs,
     }
     if (t1->auto_loaded != t2->auto_loaded)
         return !t1->auto_loaded;
+    if (preferred_program != -1 && t1->program_id != -1 && t2->program_id != -1) {
+        if ((t1->program_id == preferred_program) !=
+            (t2->program_id == preferred_program))
+            return t1->program_id == preferred_program;
+    }
     int l1 = match_lang(langs, t1->lang), l2 = match_lang(langs, t2->lang);
     if (l1 != l2)
         return l1 > l2;
@@ -529,6 +535,8 @@ struct track *select_default_track(struct MPContext *mpctx, int order,
                         (!opts->subs_with_matching_audio &&
                          mpctx->current_track[0][STREAM_AUDIO] &&
                          match_lang(langs, mpctx->current_track[0][STREAM_AUDIO]->lang));
+    int preferred_program = (type != STREAM_VIDEO && mpctx->current_track[0][STREAM_VIDEO]) ?
+                            mpctx->current_track[0][STREAM_VIDEO]->program_id : -1;
     if (tid == -2)
         return NULL;
     bool select_fallback = type == STREAM_VIDEO || type == STREAM_AUDIO;
@@ -545,7 +553,7 @@ struct track *select_default_track(struct MPContext *mpctx, int order,
             continue;
         if (duplicate_track(mpctx, order, type, track))
             continue;
-        if (!pick || compare_track(track, pick, langs, prefer_forced, mpctx->opts))
+        if (!pick || compare_track(track, pick, langs, prefer_forced, mpctx->opts, preferred_program))
             pick = track;
     }
     if (pick && !select_fallback && !(pick->is_external && !pick->no_default)