lavf: use the io_open callbacks for files opened from open_input() as well

There is no real reason to treat them differently.

libavformat/avformat.h

@@ -1237,9 +1237,9 @@ typedef struct AVFormatContext {
     /**
      * A callback for opening new IO streams.
      *
-     * Certain muxers or demuxers (e.g. for various playlist-based formats) need
-     * to open additional files during muxing or demuxing. This callback allows
-     * the caller to provide custom IO in such cases.
+     * Whenever a muxer or a demuxer needs to open an IO stream (typically from
+     * avformat_open_input() for demuxers, but for certain formats can happen at
+     * other times as well), it will call this callback to obtain an IO context.
      *
      * @param s the format context
      * @param pb on success, the newly opened IO context should be returned here

libavformat/utils.c

@@ -200,8 +200,8 @@ static int init_input(AVFormatContext *s, const char *filename,
         (!s->iformat && (s->iformat = av_probe_input_format(&pd, 0))))
         return 0;
 
-    if ((ret = avio_open2(&s->pb, filename, AVIO_FLAG_READ,
-                          &s->interrupt_callback, options)) < 0)
+    ret = s->io_open(s, &s->pb, filename, AVIO_FLAG_READ, options);
+    if (ret < 0)
         return ret;
     if (s->iformat)
         return 0;