lavc: add format field to AVFrame

The format is a per-frame property, having it in AVFrame simplify the
operation of extraction of that information, since avoids the need to
access the codec/stream context.

libavcodec/avcodec.h

@@ -1025,6 +1025,15 @@ typedef struct AVPanScan{
      * - decoding: Read by user.\
      */\
     int width, height;\
+\
+    /**\
+     * format of the frame, -1 if unknown or unset\
+     * It should be cast to the corresponding enum (enum PixelFormat\
+     * for video, enum AVSampleFormat for audio)\
+     * - encoding: unused\
+     * - decoding: Read by user.\
+     */\
+    int format;\
 
 
 #define FF_QSCALE_TYPE_MPEG1 0