core: avoid deselecting and reselecting stream needlessly

The core deselected all streams on initialization, and then selected the
streams it actually wanted. This was no problem for
demux_mkv/demux_lavf, but old demuxers (like demux_asf) could lose some
packets. The problem is that these demuxers can buffer some data on
initialization, which then is flushed on track switching. Fix this by
explicitly avoiding deselecting a wanted stream.

core/mplayer.c

@@ -443,8 +443,12 @@ static void preselect_demux_streams(struct MPContext *mpctx)
 {
     // Disable all streams, just to be sure no unwanted streams are selected.
     for (int n = 0; n < mpctx->num_sources; n++) {
-        for (int type = 0; type < STREAM_TYPE_COUNT; type++)
+        for (int type = 0; type < STREAM_TYPE_COUNT; type++) {
-            demuxer_switch_track(mpctx->sources[n], type, NULL);
+            struct track *track = mpctx->current_track[type];
+            if (!(track && track->demuxer == mpctx->sources[n] &&
+                  demuxer_stream_is_selected(track->demuxer, track->stream)))
+                demuxer_switch_track(mpctx->sources[n], type, NULL);
+        }
     }
 
     for (int type = 0; type < STREAM_TYPE_COUNT; type++) {

demux/demux.c

@@ -1214,6 +1214,10 @@ void demuxer_switch_track(struct demuxer *demuxer, enum stream_type type,
 {
     assert(!stream || stream->type == type);
 
+    // don't flush buffers if stream is already selected
+    if (stream && demuxer_stream_is_selected(demuxer, stream))
+        return;
+
     int old_id = demuxer->ds[type]->id;
 
     // legacy