sub: don't check for duplicates on sub conversion

This mirrors commit "sub: remove check_duplicate_plaintext_event()".
That code was basically duplicated. In general, this code is still
needed when doing conversion during demuxing (mostly because you can
seek during demuxing, which will cause duplicate events by replaying).

sub/sd.h

@@ -27,6 +27,10 @@ struct sd {
     struct ass_library *ass_library;
     struct ass_renderer *ass_renderer;
 
+    // If false, try to remove multiple subtitles.
+    // (Only for decoders which have accept_packets_in_advance set.)
+    bool no_remove_duplicates;
+
     // Set by sub converter
     const char *output_codec;
     char *output_extradata;