avformat: Do not search through the AVOption table for a option not in it repeatedly on each packet

This search takes alot of time especially when compared with small packets

46631 decicycles -> 15719 decicycles in read_frame_internal() for amr-nb in 3gp

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/internal.h

@@ -183,6 +183,11 @@ typedef struct FFFormatContext {
      * Set if chapter ids are strictly monotonic.
      */
     int chapter_ids_monotonic;
+
+    /**
+     * Contexts and child contexts do not contain a metadata option
+     */
+    int metafree;
 } FFFormatContext;
 
 static av_always_inline FFFormatContext *ffformatcontext(AVFormatContext *s)