libxvid: add working lumimasking and variance AQ

The old implementation is unusable due to changes in the Xvid API.
Further fixes by Michael Niedermayer <michaelni@gmx.at>.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
libxvid: Add SSIM displaying through a libxvidcore plugin

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>