swscale/slice: fix init of 32 bpc planes

In input.c and output.c and many other places, swscale follows the rule of using
15-bit intermediate if output bpc is <= 8, and 19-bit (inside int32_t)
intermediate otherwise. See e.g. the comments on hyScale() on
swscale_internal.h. These are also the coefficients that yuv2gbrpf32_full_X_c()
is using.

In contrast to this, the plane init code in slice.c (function fill_ones) is
assuming that we use 35-bit intermediates (inside 64-bit integers) for this
case, seemingly added by commit b4967fc71c63eae8cd96f9c46cd3e1fbd705bbf9 with
no further justification.

This causes a mismatch whenever the implicitly initialized plane contents leak
out to the output, e.g. when converting from grayscale to RGB.

Fixes: ticket #10716
Signed-off-by: Niklas Haas <git@haasn.dev>
Sponsored-by: Sovereign Tech Fund

libswscale/slice.c

@@ -194,14 +194,10 @@ static void fill_ones(SwsSlice *s, int n, int bpc)
     for (i = 0; i < 4; ++i) {
         size = s->plane[i].available_lines;
         for (j = 0; j < size; ++j) {
-            if (bpc == 16) {
+            if (bpc >= 16) {
                 end = (n>>1) + 1;
                 for (k = 0; k < end; ++k)
                     ((int32_t*)(s->plane[i].line[j]))[k] = 1<<18;
-            } else if (bpc == 32) {
-                end = (n>>2) + 1;
-                for (k = 0; k < end; ++k)
-                    ((int64_t*)(s->plane[i].line[j]))[k] = 1LL<<34;
             } else {
                 end = n + 1;
                 for (k = 0; k < end; ++k)