GitLab

Adds dependency to libavfilter, which provides many useful utilities
such as hardware encoding, mixing audio or rotating video.

Access to libavfilter is done through the MediaFilter class, which takes
a filter graph string and AVFrames as input and outputs filtered
AVFrames.

Adds unit tests for MediaFilter.

Change-Id: I0bf3869a9f19ad9cbbe1fc5e2d37eb83ece66ccc
Reviewed-by: Sebastien Blin <sebastien.blin@savoirfairelinux.com>