Read a variable length number our of delta and move the offset.
function readLength() {
    var byte = delta[deltaOffset++];
    var length = byte & 0x7f;
    var shift = 7;
    while (byte & 0x80) {
      byte = delta[deltaOffset++];
      length |= (byte & 0x7f) << shift;
      shift += 7;
    }
    return length;
  }