avfilter/blend_modes: Always preserve constness

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>

libavfilter/blend_modes.c

@@ -93,8 +93,8 @@ static void fn0(NAME)(const uint8_t *_top, ptrdiff_t top_linesize, \
      ptrdiff_t width, ptrdiff_t height,                       \
      FilterParams *param, double *values, int starty)         \
 {                                                                                   \
-    const PIXEL *top = (PIXEL *)_top;                                               \
-    const PIXEL *bottom = (PIXEL *)_bottom;                                         \
+    const PIXEL *top = (const PIXEL *)_top;                                         \
+    const PIXEL *bottom = (const PIXEL *)_bottom;                                   \
     PIXEL *dst = (PIXEL *)_dst;                                                     \
     const float opacity = param->opacity;                                           \
                                                                                     \

libavfilter/vf_blend.c

@@ -133,8 +133,8 @@ static void blend_expr_## name(const uint8_t *_top, ptrdiff_t top_linesize,
                                ptrdiff_t width, ptrdiff_t height,              \
                                FilterParams *param, double *values, int starty) \
 {                                                                              \
-    const type *top = (type*)_top;                                             \
-    const type *bottom = (type*)_bottom;                                       \
+    const type *top = (const type*)_top;                                       \
+    const type *bottom = (const type*)_bottom;                                 \
     type *dst = (type*)_dst;                                                   \
     AVExpr *e = param->e;                                                      \
     int y, x;                                                                  \

libavfilter/vf_blend_init.h

@@ -82,8 +82,8 @@ static void blend_normal_##name(const uint8_t *_top, ptrdiff_t top_linesize,
                                 ptrdiff_t width, ptrdiff_t height,                \
                                 FilterParams *param, double *values, int starty)  \
 {                                                                                 \
-    const type *top = (type*)_top;                                                \
-    const type *bottom = (type*)_bottom;                                          \
+    const type *top = (const type*)_top;                                          \
+    const type *bottom = (const type*)_bottom;                                    \
     type *dst = (type*)_dst;                                                      \
     const float opacity = param->opacity;                                         \
                                                                                   \