Add support for subtitles (both embedded in the video) or added through .srt or .vtt files.

mrViewer supported subtitles both embedded in the .mp4 video file or added .srt file.  This is a feature still missing in mrv2.

While embedded subtitles can be handled by FFmpegReadVideo.cpp, .srt/.vtt subtitles are supported in FFmpeg through filters, so we would need to add the avfilter library once again.