The effect works in two passes :

 - 	The screen and the volumetric sphere mask are stored in an intermediate render texture (Screen = RGB, Mask = A).
	By enabling the render texture's mipmaps and by setting the filter mode to point (no interpolation between pixels) we can achieve a virtually free pixelization of the texture. Each mip level will be twice more pixelated than the previous one.
	You can change the mask for any other at this stage (see the "Screen And Mask" shader) and the rest of the pixelization will still work with your own mask.

 - 	We then iterate through the render texture and layer by layer sample in a different mip level, according to the wanted amount of pixelization.