Update ffmpeg 2.1.3 -> 5.0.1

Old ffmpeg has a lot of vulnerabilities that have been fixed in newer
versions, see https://www.cvedetails.com/vulnerability-list/vendor_id-3611/Ffmpeg.html.

extern/ffmpeg/libavcodec/startcode.h

@@ -0,0 +1,32 @@
+/*
+ * This file is part of FFmpeg.
+ *
+ * FFmpeg is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * FFmpeg is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with FFmpeg; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/**
+ * @file
+ * Accelerated start code search function for start codes common to
+ * MPEG-1/2/4 video, VC-1, H.264/5
+ */
+
+#ifndef AVCODEC_STARTCODE_H
+#define AVCODEC_STARTCODE_H
+
+#include <stdint.h>
+
+int ff_startcode_find_candidate_c(const uint8_t *buf, int size);
+
+#endif /* AVCODEC_STARTCODE_H */