H264 start code 3/4 bytes

x264 uses 3 byte start codes where the spec says those start codes canbe used.  4 byte start codes must be used at the start of SPS, PPS,and in the first NAL of a frame.

leading_zero_8bits is a byte equal to 0x00.NOTE – The leading_zero_8bits syntax element can only be present in the first byte stream NAL unit of the bitstream, because(as shown in the syntax diagram of subclause B.1.1) any bytes equal to 0x00 that follow a NAL unit syntax structure and precedethe four-byte sequence 0x00000001 (which is to be interpreted as a zero_byte followed by a start_code_prefix_one_3bytes) willbe considered to be trailing_zero_8bits syntax elements that are part of the preceding byte stream NAL unit.