host: split write_image into GIF and MP4 (fails!)

host: split write_image into GIF and MP4 (fails!)

split write_image() into convert_to_gif() (using ImageMagick 'convert')
and convert_to_mp4() (using FFMPEG) functions.

NOTE:
since the Parallella Ubuntu ships with libav instead of ffmpeg,
and the ffmpeg/avconv binaries don't support type globs, converting
to MP4 does not work - and fails without error message.