Fix VSE subtitle export indexing

Some programs have trouble parsing .srt files if first index is 0.

While there doesn't seem to be any official specs for the .srt file
format, all resourses I could find online start indexing at 1. This
commit makes Blender's .srt exports start at 1 instead of 0.

Pull Request: https://projects.blender.org/blender/blender/pulls/107997

source/blender/editors/space_sequencer/sequencer_edit.cc

@@ -3174,7 +3174,7 @@ static int sequencer_export_subtitles_exec(bContext *C, wmOperator *op)
   Sequence *seq, *seq_next;
   Editing *ed = SEQ_editing_get(scene);
   ListBase text_seq = {0};
-  int iter = 0;
+  int iter = 1; /* Sequence numbers in .srt files are 1-indexed. */
   FILE *file;
   char filepath[FILE_MAX];