ffmpegthumbs + ffmpeg 2.9

[Harald Sitter]

heya,

ffmpeg 2.9 sees the removal of previously deprecated API. since
ffmpegthumbs hasn't had a proper maintenance pass in a while this
needs quite a bit of porting for which there is a review request
already, but someone needs to do the review [1].
given ffmpeg's aggressive deprecation policies it probably would be
handy to have a maintainer for ffmpegthumbs who checks for
deprecations every once in a while to stay ahead of breakage and make
sure the thumbnailer is of adequate quality.

[1] https://git.reviewboard.kde.org/r/126992

HS