external_files: detect locales with a region like en-US

This loads subtitle files like foo.en-US.srt with --sub-auto=exact.

To preserve the case of these locales and not convert them to e.g.
en-us, stop lower casing filenames, and instead use case insensitive
functions to check if the media filename is contained in the external
filenames. Extensions, whitelisted cover art filenames and idx files
were already being compared case insensitively.

This allows locales of any number of characters because there are
locales like gsw-u-sd-chzh.

Fixes #12372, fixes #13251.

player/external_files.c

@@ -128,15 +128,13 @@ static struct bstr guess_lang_from_filename(struct bstr name, int *lang_start)
         i--;
     }
 
-    while (i >= 0 && mp_isalpha(name.start[i])) {
+    while (i >= 0 && (mp_isalpha(name.start[i]) || name.start[i] == '-')) {
         lang_length++;
-        if (lang_length > 3)
-            return (struct bstr){NULL, 0};
         i--;
     }
 
     if (lang_length < 2 || i == 0 || name.start[i] != delimiter)
-        return (struct bstr){NULL, 0};
+        return (struct bstr){0};
 
     *lang_start = i;
     return (struct bstr){name.start + i + 1, lang_length};
@@ -162,7 +160,6 @@ static void append_dir_subtitles(struct mpv_global *global, struct MPOpts *opts,
     struct bstr f_fname = mp_iconv_to_utf8(log, f_fbname,
                                            "UTF-8-MAC", MP_NO_LATIN1_FALLBACK);
     struct bstr f_fname_noext = bstrdup(tmpmem, bstr_strip_ext(f_fname));
-    bstr_lower(f_fname_noext);
     struct bstr f_fname_trim = bstr_strip(f_fname_noext);
 
     if (f_fbname.start != f_fname.start)
@@ -185,7 +182,6 @@ static void append_dir_subtitles(struct mpv_global *global, struct MPOpts *opts,
                                               "UTF-8-MAC", MP_NO_LATIN1_FALLBACK);
         // retrieve various parts of the filename
         struct bstr tmp_fname_noext = bstrdup(tmpmem2, bstr_strip_ext(dename));
-        bstr_lower(tmp_fname_noext);
         struct bstr tmp_fname_ext = bstr_get_ext(dename);
         struct bstr tmp_fname_trim = bstr_strip(tmp_fname_noext);
 
@@ -217,13 +213,13 @@ static void append_dir_subtitles(struct mpv_global *global, struct MPOpts *opts,
         // higher prio -> auto-selection may prefer it (0 = not loaded)
         int prio = 0;
 
-        if (bstrcmp(tmp_fname_trim, f_fname_trim) == 0)
+        if (bstrcasecmp(tmp_fname_trim, f_fname_trim) == 0)
             prio |= 32; // exact movie name match
 
         bstr lang = {0};
         int start = 0;
         lang = guess_lang_from_filename(tmp_fname_trim, &start);
-        if (bstr_startswith(tmp_fname_trim, f_fname_trim)) {
+        if (bstr_case_startswith(tmp_fname_trim, f_fname_trim)) {
             if (lang.len && start == f_fname_trim.len)
                 prio |= 16; // exact movie name + followed by lang