Neatness also cull extra flecks

The neatness currently moves all the flecks onto their nearest grid point, stacking them up. This makes a lot of overdraw, potentially. And as the flecks may be different variants, or slightly different sizes, you often can visually see them stacked up. And depending on the settings they can have high contrast and look strange--as demonstrated in the preview stream from Mm.

We can lessen the visual impact of this with certain settings, but it seems a waste to render lots of flecks on top of one another. A setting to cull those when multiple are on the same grid spot would help with rendering performance, and to adjust the look to appear more uniform and neat.