lavfi/video: set output frame w/h properties in default_start_frame()

Use the same values of the video output link.

Avoid the need to override the default_start_frame() with an ad-hoc
start_frame() callback.

In particular, fix the super2xsai filter which was setting the
input w/h values in the output.

libavfilter/video.c

@@ -164,6 +164,8 @@ static void default_start_frame(AVFilterLink *inlink, AVFilterBufferRef *picref)
     if (outlink) {
         outlink->out_buf = ff_get_video_buffer(outlink, AV_PERM_WRITE, outlink->w, outlink->h);
         avfilter_copy_buffer_ref_props(outlink->out_buf, picref);
+        outlink->out_buf->video->w = outlink->w;
+        outlink->out_buf->video->h = outlink->h;
         ff_start_frame(outlink, avfilter_ref_buffer(outlink->out_buf, ~0));
     }
 }