src/ : Move to a variable length header buffer

Previously, the `psf->header` buffer was a fixed length specified by
`SF_HEADER_LEN` which was set to `12292`. This was problematic for
two reasons; this value was un-necessarily large for the majority
of files and too small for some others.

Now the size of the header buffer starts at 256 bytes and grows as
necessary up to a maximum of 100k.
sf_writef_double	(SNDFILE *sndfile, const double *ptr, sf_count_t frames)
{	SF_PRIVATE 	*psf ;
	sf_count_t	count ;

	VALIDATE_SNDFILE_AND_ASSIGN_PSF (sndfile, psf, 1) ;

	if (psf->file.mode == SFM_READ)
	{	psf->error = SFE_NOT_WRITEMODE ;
		return 0 ;
		} ;

	if (psf->write_double == NULL || psf->seek == NULL)
	{	psf->error = SFE_UNIMPLEMENTED ;
		return 0 ;
		} ;

	if (psf->last_op != SFM_WRITE)
		if (psf->seek (psf, SFM_WRITE, psf->write_current) < 0)
			return 0 ;

	if (psf->have_written == SF_FALSE && psf->write_header != NULL)
		psf->write_header (psf, SF_FALSE) ;
	psf->have_written = SF_TRUE ;

	count = psf->write_double (psf, ptr, frames * psf->sf.channels) ;

	psf->write_current += count / psf->sf.channels ;

	psf->last_op = SFM_WRITE ;

	if (psf->write_current > psf->sf.frames)
	{	psf->sf.frames = psf->write_current ;
		psf->dataend = 0 ;
		} ;

	if (psf->auto_header && psf->write_header != NULL)
		psf->write_header (psf, SF_TRUE) ;

	return count / psf->sf.channels ;
} /* sf_writef_double */

sf_writef_double	(SNDFILE *sndfile, const double *ptr, sf_count_t frames)
{	SF_PRIVATE 	*psf ;
	sf_count_t	count ;

	VALIDATE_SNDFILE_AND_ASSIGN_PSF (sndfile, psf, 1) ;

	if (psf->file.mode == SFM_READ)
	{	psf->error = SFE_NOT_WRITEMODE ;
		return 0 ;
		} ;

	if (psf->write_double == NULL || psf->seek == NULL)
	{	psf->error = SFE_UNIMPLEMENTED ;
		return 0 ;
		} ;

	if (psf->last_op != SFM_WRITE)
		if (psf->seek (psf, SFM_WRITE, psf->write_current) < 0)
			return 0 ;

	if (psf->have_written == SF_FALSE && psf->write_header != NULL)
		psf->write_header (psf, SF_FALSE) ;
	psf->have_written = SF_TRUE ;

	count = psf->write_double (psf, ptr, frames * psf->sf.channels) ;

	psf->write_current += count / psf->sf.channels ;

	psf->last_op = SFM_WRITE ;

	if (psf->write_current > psf->sf.frames)
	{	psf->sf.frames = psf->write_current ;
		psf->dataend = 0 ;
		} ;

	if (psf->auto_header && psf->write_header != NULL)
		psf->write_header (psf, SF_TRUE) ;

	return count / psf->sf.channels ;
} /* sf_writef_double */
