libvpx: Pull from upstream

source/libvpx/third_party/libmkv/EbmlBufferWriter.c

+// #include <strmif.h>
+#include "EbmlBufferWriter.h"
+#include "EbmlWriter.h"
+// #include <cassert>
+// #include <limits>
+// #include <malloc.h>  //_alloca
+#include <stdlib.h>
+#include <wchar.h>
+#include <string.h>
+
+void Ebml_Write(EbmlGlobal *glob, const void *buffer_in, unsigned long len) {
+  unsigned char *src = glob->buf;
+  src += glob->offset;
+  memcpy(src, buffer_in, len);
+  glob->offset += len;
+}
+
+static void _Serialize(EbmlGlobal *glob, const unsigned char *p, const unsigned char *q) {
+  while (q != p) {
+    --q;
+
+    unsigned long cbWritten;
+    memcpy(&(glob->buf[glob->offset]), q, 1);
+    glob->offset++;
+  }
+}
+
+void Ebml_Serialize(EbmlGlobal *glob, const void *buffer_in, unsigned long len) {
+  // assert(buf);
+
+  const unsigned char *const p = (const unsigned char *)(buffer_in);
+  const unsigned char *const q = p + len;
+
+  _Serialize(glob, p, q);
+}
+
+
+void Ebml_StartSubElement(EbmlGlobal *glob, EbmlLoc *ebmlLoc, unsigned long class_id) {
+  Ebml_WriteID(glob, class_id);
+  ebmlLoc->offset = glob->offset;
+  // todo this is always taking 8 bytes, this may need later optimization
+  unsigned long long unknownLen =  0x01FFFFFFFFFFFFFFLLU;
+  Ebml_Serialize(glob, (void *)&unknownLen, 8); // this is a key that says lenght unknown
+}
+
+void Ebml_EndSubElement(EbmlGlobal *glob, EbmlLoc *ebmlLoc) {
+  unsigned long long size = glob->offset - ebmlLoc->offset - 8;
+  unsigned long long curOffset = glob->offset;
+  glob->offset = ebmlLoc->offset;
+  size |=  0x0100000000000000LLU;
+  Ebml_Serialize(glob, &size, 8);
+  glob->offset = curOffset;
+}
+