swscale: fix aspect-ratio problem properly

works with transform filter now too, fixes #7437 again.
(cherry picked from commit 0330378c55b8b940f6de82adb59cc32c5b3da12a)

modules/video_filter/swscale.c

@@ -208,8 +208,8 @@ static int OpenScaler( vlc_object_t *p_this )
              (char *)&p_filter->fmt_out.video.i_chroma,
              ppsz_mode_descriptions[i_sws_mode] );
 
-    p_filter->fmt_out.video.i_sar_num = p_filter->fmt_in.video.i_sar_num;
-    p_filter->fmt_out.video.i_sar_den = p_filter->fmt_in.video.i_sar_den;
+    p_filter->fmt_out.video.i_sar_num = p_filter->fmt_in.video.i_sar_num * p_filter->fmt_in.video.i_width / p_filter->fmt_out.video.i_width;
+    p_filter->fmt_out.video.i_sar_den = p_filter->fmt_in.video.i_sar_den * p_filter->fmt_in.video.i_height / p_filter->fmt_out.video.i_height;
 
     return VLC_SUCCESS;
 }