avfilter/vf_bm3d: Dont round MSE2SSE to an integer

Fixes: CID1439581 Result is not floating-point

Sponsored-by: Sovereign Tech Fund
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
(cherry picked from commit ec18ec9fc1)
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavfilter/vf_bm3d.c

@@ -273,7 +273,7 @@ static void do_block_matching_multi(BM3DContext *s, const uint8_t *src, int src_
                                     int r_y, int r_x, int plane, int jobnr)
 {
     SliceContext *sc = &s->slices[jobnr];
-    double MSE2SSE = s->group_size * s->block_size * s->block_size * src_range * src_range / (s->max * s->max);
+    double MSE2SSE = s->group_size * s->block_size * s->block_size * src_range * src_range / (double)(s->max * s->max);
     double distMul = 1. / MSE2SSE;
     double th_sse = th_mse * MSE2SSE;
     int index = sc->nb_match_blocks;