Blender Git Loki

CMake/Find packages: add FindFFmpeg.cmake module.

The behaviour is a bit different from other find modules:
`FFMPEG_FIND_COMPONENTS` should contain the
list of required libraries before calling `find_package(FFmpeg)`.
If not set, a default set of most common libraries is searched for:
`avcodec avdevice avfilter avformat avutil`
http://ffmpeg.org/download.html

Ref D8855

Reviewed By: brecht

Differential Revision: https://developer.blender.org/D8918