demux: read sparse still-image streams lazily

still_image streams (AV_DISPOSITION_STILL_IMAGE) consist of sparse
frames that can be minutes apart. When such a stream was selected
alongside an eager audio/video stream, its empty packet queue forced
the demuxer to keep reading ahead, buffering the whole file trying to
chase min_secs of "video". Read them lazily when other eager streams
exist, so playback drives read-ahead instead.

demux/demux.c

@@ -913,13 +913,13 @@ static void update_stream_selection_state(struct demux_internal *in,
         any_streams |= s->selected;
     }
 
-    // Subtitles are only eagerly read if there are no other eagerly read
+    // Subtitles and sparse still-image video are only eagerly read if there are
-    // streams.
+    // no other eagerly read streams.
     if (any_av_streams) {
         for (int n = 0; n < in->num_streams; n++) {
             struct demux_stream *s = in->streams[n]->ds;
 
-            if (s->type == STREAM_SUB)
+            if (s->type == STREAM_SUB || s->still_image)
                 s->eager = false;
         }
     }