These casts cast const away temporarily; they are safe, because
the pointers that are initialized point to const data. But this
is nevertheless not nice and leads to warnings when using
-Wcast-qual. blend_modes.c generates 546 (2*39*7) such warnings
which is the majority of such warnings for FFmpeg as a whole.
vf_blend.c and vf_blend_init.h also use this pattern;
they have also been changed.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>