lavfi/minterpolate: fix blending calc issue.

the right blending calc is:
(alpha * Frame_2 + (MAX - alpha) * Frame_1 + 512) >> 10

Signed-off-by: Jun Zhao <mypopydev@gmail.com>

libavfilter/vf_minterpolate.c

@@ -1122,8 +1122,8 @@ static void interpolate(AVFilterLink *inlink, AVFrame *avf_out)
                 for (y = 0; y < height; y++) {
                     for (x = 0; x < width; x++) {
                         avf_out->data[plane][x + y * avf_out->linesize[plane]] =
-                                          alpha  * mi_ctx->frames[2].avf->data[plane][x + y * mi_ctx->frames[2].avf->linesize[plane]] +
-                            ((ALPHA_MAX - alpha) * mi_ctx->frames[1].avf->data[plane][x + y * mi_ctx->frames[1].avf->linesize[plane]] + 512) >> 10;
+                            (alpha  * mi_ctx->frames[2].avf->data[plane][x + y * mi_ctx->frames[2].avf->linesize[plane]] +
+                             (ALPHA_MAX - alpha) * mi_ctx->frames[1].avf->data[plane][x + y * mi_ctx->frames[1].avf->linesize[plane]] + 512) >> 10;
                     }
                 }
             }