multimedia/ffmpeg: update to 2.8.10

This is a direct commit to 2016Q4. /head switched to 3.x in r428352.

Changes: https://git.ffmpeg.org/gitweb/ffmpeg.git/blob/n2.8.10:/Changelog
ABI: https://abi-laboratory.pro/tracker/timeline/ffmpeg/
Approved by: ports-secteam (junovitch)