Improvements to CameraFrame.vignette

[animech-sijo]

The current implementation of vignette could to with some improvements:

• Composite the vignette in HDR. An optical vignette lowers the exposure around the edges, and isn't just composited over the final image. When working with blooming HDR lights, the current implementation produces grey-ish highlights.
• Implement a circular vignette shape. An optical vignette is circular due to the shape of the lens/aperture. The current implementation is dependent on the aspect ratio of the canvas, which is not how a real vignette works. I do see the artistic value in having a configurable shape, but it should also be able to produce a circle.

Here I've exaggerated the vignette intensity to show the issues. The squares have a material with emissiveIntensity set to 100:

[mvaligursky]

Fair points. The vignette we have is an overlay one. Perhaps we should leave it as is, and add some lens one which is more physically based.