cdg: Try to detect keyframes.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/cdg.c

@@ -53,6 +53,10 @@ static int read_packet(AVFormatContext *s, AVPacket *pkt)
 
     pkt->stream_index = 0;
     pkt->dts=pkt->pts= s->streams[0]->cur_dts;
+
+    if(ret>5 && (pkt->data[0]&0x3F) == 9 && (pkt->data[1]&0x3F)==1 && !(pkt->data[2+2+1] & 0x0F)){
+        pkt->flags = AV_PKT_FLAG_KEY;
+    }
     return ret;
 }