avformat/matroskadec: set all frames in a keyframe simple box as keyframes

And not just the first, as it was being done for lacing. The spec states that the
flag applies to everything inside the simple box.

Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/matroskadec.c

@@ -4224,7 +4224,7 @@ static int matroska_parse_block(MatroskaDemuxContext *matroska, AVBufferRef *buf
         } else {
             res = matroska_parse_frame(matroska, track, st, buf, out_data,
                                        out_size, timecode, lace_duration,
-                                       pos, !n ? is_keyframe : 0,
+                                       pos, is_keyframe,
                                        blockmore, nb_blockmore,
                                        discard_padding);
             if (res)