video: mac hardware acceleration

Adds VideoToolbox and VDA hardware accelerations. VideoToolbox
supports H.264, H.263 and MPEG4, while VDA only supports H.264.

VDA is implemented in case libav is used instead of FFmpeg, as
only the latter implements VideoToolbox. This being said, Ring
will prefer VideoToolbox.

VideoToolbox is OSX 10.8+ and iOS 8+. VDA is OSX 10.6.3+.

Both have their respective configure switches.

Change-Id: I588fcbb92809a9d6a56bb9b6a7ac3a59874c0186
Tuleap: #1090
Reviewed-by: Anthony Léonard <anthony.leonard@savoirfairelinux.com>