amask_type
const amask_type* m_mask;
typedef typename amask_type::cover_type cover_type;
memset(&m_span[0], amask_type::cover_full, len * sizeof(cover_type));
pixfmt_amask_adaptor(pixfmt_type& pixf, const amask_type& mask) :
void attach_alpha_mask(const amask_type& mask) { m_mask = &mask; }