/*
 * Pass raw PCM data to generate a signature.
 *
 * Note: MusicBrainz::TRM#pcm_data must be called before this function.
 *
 * Returns true if enough data has been sent to generate a signature,
 * and false if more data is needed.
 *
 * Example:
 *   trm.generate_signature buf
 *
 */
static VALUE mb_trm_gen_sig(VALUE self, VALUE buf) {