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Filters were reverted for the release so the binaries wouldn't include
libavfilter (which wasn't yet used).

This reverts commit 1e6a4716.

Change-Id: Ib9790a544b442066265b3fe1ea96313be35eff62
Reviewed-by: Sebastien Blin <sebastien.blin@savoirfairelinux.com>

This reverts commit ea2933d4.

Change-Id: I624c07255f60186a0d1e6610a58956775f91b342

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>