message: vertexcodec: Use zigzag encoding for deltas
  This doesn't make much of a difference for post-deflate size - this
  makes sense since deflate cares about the number of unique values, not
  really about the values themselves - but this will make it easier to
  analyze the data and compress using integer packing.
diff: @@ -15,6 +15,11 @@ namespace meshopt
  const size_t kVertexBlockSize = 256;
  +inline unsigned char zigzag8(unsigned char v)
  +{
  + return (v >> 7) | ((v ^ -(v >> 7)) << 1);
  +}
  +
  inline int bits(unsigned char v)
  {
  int result = 0;
  @@ -69,7 +74,7 @@ static unsigned char* encodeVertexBlock(unsigned char* data, const unsigned char
  }
  }
  - unsigned char delta = vertex_data[vertex_offset] - p;
  + unsigned char delta = zigzag8(vertex_data[vertex_offset] - p);
  if (!uniqb[k][delta])
  {
  @@ -158,7 +163,7 @@ static unsigned char* encodeVertexBlock(unsigned char* data, const unsigned char
  }
  }
  - unsigned char delta = vertex_data[vertex_offset] - p;
  + unsigned char delta = zigzag8(vertex_data[vertex_offset] - p);
  *data++ = delta;
  vertex_offset += vertex_size;
  