static void monoblack_to_gray(AVPicture *dst, AVPicture *src,
static void monoblack_to_gray(AVPicture *dst, const AVPicture *src,
static void monoblack_to_gray(AVPicture *dst, AVPicture *src,                               int width, int height){    mono_to_gray(dst, src, width, height, 0x00);}