Add a comment to an initialized theora_comment structure
\param tc A previously initialized theora comment structure
\param comment A null-terminated string encoding the comment in the form
"TAG=the value"
Neither theora_comment_add() nor theora_comment_add_tag() support
comments containing null values, although the bitstream format
supports this.
Report whether a theora packet is a header or not
This function does no verification beyond checking the header
flag bit so it should not be used for bitstream identification;
use theora_decode_header() for that.