player: add stream selection by ffmpeg index

Apparently using the stream index is the best way to refer to the same
streams across multiple FFmpeg-using programs, even if the stream index
itself is rarely meaningful in any way.

For Matroska, there are some possible problems, depending how FFmpeg
actually adds streams. Normally they seem to match though.

1 files changed, 2 insertions, 2 deletions

diff --git a/player/core.h b/player/core.h
index 4d4142000c..f883ae2882 100644
--- a/player/core.h
+++ b/player/core.h
@@ -96,8 +96,8 @@ struct track {
     // IDs coming from demuxers or container files.
     int user_tid;
 
-    // Same as stream->demuxer_id. -1 if not set.
-    int demuxer_id;
+    int demuxer_id; // same as stream->demuxer_id. -1 if not set.
+    int ff_index; // same as stream->ff_index, or 0.
 
     char *title;
     bool default_track;