Add dithering to Gaussian Blur

[Chain83]

Problem: Blurring causes banding/posterization (for 8-bit/channel files). It is most apparent with very heavy (large radius) blurs.

Gaussian blur is the most commonly used blur filter. I suggest adding a dithering option to it (for adding a diffusion dither). A simple checkbox would do:

Side notes:

If someone has visible banding, they might try to use gaussian blur to blend the steps to fix it. This will often not work well (if in 8-bit) since Gaussian Blur isn't dithered. A workaround is to use the old Spatter filter set to Smoothness 1 (after the gaussian blur). Very few people would be aware of this, so it would be better if Gaussian Blur did a better job! 🙂