sub: by default, don't render timestamps after video EOF

This fixes a long-standing apparent issue where mpv would display the last
frame with no subtitles at EOF. This is caused by sub rendering switching from
video timestamps to audio timestamps when the video ends, and audio streams
often running past the timestamp of the last video frame. However, authoring
tools (most notably Aegisub) don't tend to provide easy ways to add meaningful
subtitles after the end of the video, so this is rarely actually useful.

options/options.c

@@ -275,6 +275,7 @@ const struct m_sub_options mp_subtitle_sub_opts = {
         {"sub", OPT_SUBSTRUCT(sub_style, sub_style_conf)},
         {"sub-clear-on-seek", OPT_FLAG(sub_clear_on_seek)},
         {"teletext-page", OPT_INT(teletext_page), M_RANGE(1, 999)},
+        {"sub-past-video-end", OPT_FLAG(sub_past_video_end)},
         {0}
     },
     .size = sizeof(OPT_BASE_STRUCT),

options/options.h

@@ -107,6 +107,7 @@ struct mp_subtitle_opts {
     int ass_justify;
     int sub_clear_on_seek;
     int teletext_page;
+    int sub_past_video_end;
 };
 
 struct mp_sub_filter_opts {