hxxx_helper is a H264/HEVC helper used by mediacodec and videtoolbox. For
mediacodec, it's used to convert (and validate) xvcC to AnnexB or used to
detect SPS/PPS change when decoding AnnexB. For videotoolbox, it's used to
validate avcC or to convert AnnexB to avcC (and detect SPS/PPS change) when
decoding AnnexB.