fix transposed arguments to calloc()

calloc() wants the number of members followed by the
size of each member. obviously the product is the same
either way, but there could be alignment problems if
the arguments are transposed, as they were here.

src/lib/render.c

@@ -580,7 +580,7 @@ int ncplane_mergedown(ncplane* restrict src, ncplane* restrict dst,
     return -1;
   }
   const int totalcells = dst->leny * dst->lenx;
-  nccell* rendfb = calloc(sizeof(*rendfb), totalcells);
+  nccell* rendfb = calloc(totalcells, sizeof(*rendfb));
   const size_t crenderlen = sizeof(struct crender) * totalcells;
   struct crender* rvec = malloc(crenderlen);
   if(!rendfb || !rvec){