options: add --vlang switch

For symmetry with --alang and --slang. 100% useless, but why not?

DOCS/man/options.rst

@@ -30,6 +30,9 @@ Track Selection
         - ``mpv --slang=jpn example.mkv`` plays a Matroska file with Japanese
           subtitles.
 
+``--vlang=<...>``
+    Equivalent to ``--alang`` and ``--slang``, for video tracks.
+
 ``--aid=<ID|auto|no>``
     Select audio track. ``auto`` selects the default, ``no`` disables audio.
     See also ``--alang``. mpv normally prints available audio tracks on the

options/options.c

@@ -365,6 +365,7 @@ const m_option_t mp_opts[] = {
     OPT_ALIAS("audio", "aid"),
     OPT_STRINGLIST("alang", stream_lang[STREAM_AUDIO], 0),
     OPT_STRINGLIST("slang", stream_lang[STREAM_SUB], 0),
+    OPT_STRINGLIST("vlang", stream_lang[STREAM_VIDEO], 0),
     OPT_FLAG("track-auto-selection", stream_auto_sel, 0),
 
     OPT_STRING("lavfi-complex", lavfi_complex, UPDATE_LAVFI_COMPLEX),