Respect timestamps when extracting subtitles

MediaBrowser.MediaEncoding/Subtitles/SubtitleEncoder.cs

@@ -570,7 +570,7 @@ namespace MediaBrowser.MediaEncoding.Subtitles
 
             var processArgs = string.Format(
                 CultureInfo.InvariantCulture,
-                "-i {0} -map 0:{1} -an -vn -c:s {2} \"{3}\"",
+                "-i {0} -copyts -map 0:{1} -an -vn -c:s {2} \"{3}\"",
                 inputPath,
                 subtitleStreamIndex,
                 outputCodec,