Determines size of an ID3v2 tag on buffer containing at least ID3v2 header, i.e. at least #GST_TAG_ID3V2_HEADER_SIZE (10) bytes;
buffer holding ID3v2 tag (or at least the start of one)
Size of tag, or 0 if header is invalid or too small.
Determines size of an ID3v2 tag on buffer containing at least ID3v2 header, i.e. at least #GST_TAG_ID3V2_HEADER_SIZE (10) bytes;