WIP: Improved spotlight effect

One more of those tiny little things that you can't unsee once you see it.

Questions:

• Maybe I should use a noise texture instead of procedural noise? If so, what is the best place in the code to put this texture?

• As you can see, it's not possible to eliminate all banding in the shutdown dialog, because the lightbox is a gradient with an alpha rather than a gradient directly applied to the image below. In mutter, I could use the bare minimum ±0.5/255 of noise to eliminate all banding in the Overview, but with the shutdown dialog I had to add so much that the gradient is noticeably noisy if you squint, yet still there is banding, although not as jarring. Is it somehow possible to apply the vignette directly to what's below it?

Before:

After:

Edit: sorry for the rant, removed it.