mediacodec/videotoolbox: add hxxx_helper
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.
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