Nikita Churaev authored Mar 07, 2018 and Georges Basile Stavracas Neto committed Dec 12, 2018

The shader used for computing a vignette currently has two
problems:

 * The math is wrong such that the vignette isn't stretched
   across the whole actor and so ends abruptly
 * There is noticeable banding in its gradient

This commit corrects both problems by fixing the computing
and introducing noise dithering.