avcodec/h264_parser: heuristically detect non marked keyframes

Fixes Ticket3083
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264_parser.c

@@ -309,6 +309,9 @@ static inline int parse_nal_units(AVCodecParserContext *s,
             h->sps       = *h->sps_buffers[h->pps.sps_id];
             h->frame_num = get_bits(&h->gb, h->sps.log2_max_frame_num);
 
+            if(h->sps.ref_frame_count <= 1 && h->pps.ref_count[0] <= 1 && s->pict_type == AV_PICTURE_TYPE_I)
+                s->key_frame = 1;
+
             avctx->profile = ff_h264_get_profile(&h->sps);
             avctx->level   = h->sps.level_idc;