doc/encoders: Add libxvid doc
libxvid: add working lumimasking

The old implementation does not specify the param and is not usable.
libxvid: Add SSIM displaying through an libxvidcore plugin
libxvid: cosmetics: Realign the code
libxvid: Reduce the size of an array

It is not possible to have a plugin[] array with 4 or more elements.