Fix build with ffmpeg-3.x

Import slightly modified version from upstream
https://git.reviewboard.kde.org/r/126992/

With this patch the port still works with ffmpeg-2.x.
PORTREVISION is being bumped for two reasons: the dependencies have been
changed (aka "make stage-qa happy") but also the ffmpeg3 patch causes the
library to change (it links against libavfilter at the very least)

PR: 214194
Submitted by: Matthew Rezny <matthew@reztek.cz>
Reported by: jbeich
Approved by: rakuco (mentor)