mplayer: don't auto-load explicitly loaded subtitle files

Even if a subtitle was explicitly loaded with -sub, it was still auto-
loaded (if auto-loading applied to that file). Fix this by explicitly
checking whether a file is already loaded.

The check is maximal naive and just compares the filenames as strings.
The change in find_subfiles.c is so that "-sub something.ass" happens to
work (auto-loading prepended a "./" to it, so the naive filename
comparison check didn't work).

mpvcore/mplayer.c

@@ -3957,9 +3957,15 @@ static void open_subtitles_from_options(struct MPContext *mpctx)
         char **tmp = find_text_subtitles(mpctx->opts, mpctx->filename);
         int nsub = MP_TALLOC_ELEMS(tmp);
         for (int i = 0; i < nsub; i++) {
-            struct track *track = mp_add_subtitles(mpctx, tmp[i]);
+            char *filename = tmp[i];
+            for (int n = 0; n < mpctx->num_sources; n++) {
+                if (strcmp(mpctx->sources[n]->stream->url, filename) == 0)
+                    goto skip;
+            }
+            struct track *track = mp_add_subtitles(mpctx, filename);
             if (track)
                 track->auto_loaded = true;
+        skip:;
         }
         talloc_free(tmp);
     }

sub/find_subfiles.c

@@ -172,6 +172,10 @@ static void append_dir_subtitles(struct MPOpts *opts,
                 MP_GROW_ARRAY(*slist, *nsub);
                 struct subfn *sub = *slist + (*nsub)++;
 
+                // annoying and redundant
+                if (strncmp(subpath, "./", 2) == 0)
+                    subpath += 2;
+
                 sub->priority = prio;
                 sub->fname    = subpath;
             } else