avfilter/scale_vt: implement negative width/height aspect ratio sizing

Signed-off-by: Koushik Dutta <koushd@gmail.com>
Signed-off-by: Zhao Zhili <zhilizhao@tencent.com>

libavfilter/vf_scale_vt.c

@@ -189,6 +189,8 @@ static int scale_vt_config_output(AVFilterLink *outlink)
     if (err < 0)
         return err;
 
+    ff_scale_adjust_dimensions(inlink, &s->output_width, &s->output_height, 0, 1);
+
     outlink->w = s->output_width;
     outlink->h = s->output_height;