ffms: Various improvements

ffms: Various improvements

 * Drop the MinGW Unicode workarounds. Those were required at the time
   Windows Unicode support was added to x264 but the underlying problem
   has since been fixed in FFMS.

 * Use FFMS_IndexBelongsToFile() as an additional sanity check when reading
   an index file to ensure that it belongs to the current source video.

 * Upgrade to the new API to prevent deprecation warnings when compiling.

 * Fix a resource leak that would occur if FFMS_GetFirstTrackOfType() or
   FFMS_CreateVideoSource() failed.

 * Minor string handling adjustments related to progress reporting.

This increases the FFMS version requirement from 2.16.2 to 2.21.0.