Implement better dithering

[Rodrigodd]

Dithering is used to avoid banding artifacts in the smooth color gradient of the background. The current implementation of dithering were copied from Bevy's implementation. Not sure if I implemented it wrong, but it causes stripes for some color shadings.

It may be better change the dithering implementation. I have some done some tests in notebooks/dither.html.

The current implementation is in the shader script, more specifically here.