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

[ggarra13]

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.