Set AV_PKT_FLAG_KEY in avcodec_decode_video.

This fixes decoding of MPNG, which currently is incorrectly decoded as
CorePNG delta frames.
CorePNG can not be correctly decoded via this old API either way.

Originally committed as revision 19079 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/utils.c

@@ -557,6 +557,8 @@ int attribute_align_arg avcodec_decode_video(AVCodecContext *avctx, AVFrame *pic
     av_init_packet(&avpkt);
     avpkt.data = buf;
     avpkt.size = buf_size;
+    // HACK for CorePNG to decode as normal PNG by default
+    avpkt.flags = AV_PKT_FLAG_KEY;
 
     return avcodec_decode_video2(avctx, picture, got_picture_ptr, &avpkt);
 }