Fluid simulation - apply dithering on all layers, not only bloom

Dithering is a method used to prevent color banding (https://www.anisopteragames.com/how-to-fix-color-banding-with-dithering/)

In the current code, dithering is only applied to the bloom layer, but the simulation being composed of smooth gradients anyway, it should probably be applied even if bloom is disabled, on the final frame.

context: https://github.com/google/bigwheels/pull/20#discussion_r1060683083