sub: never set VSFilter aspect if the ASS subtitle is converted

When e.g. converting SRT to ASS, we certainly don't want them stretched
by video aspect ratio, even if that's necessary for native ASS
subtitles.

Annoying weird details...

sub/sd.h

@@ -16,6 +16,10 @@ struct sd {
     char *extradata;
     int extradata_len;
 
+    // Set to !=NULL if the input packets are being converted from another
+    // format.
+    const char *converted_from;
+
     // Video resolution used for subtitle decoding. Doesn't necessarily match
     // the resolution of the VO, nor does it have to be the OSD resolution.
     int sub_video_w, sub_video_h;