Fix color greyscale colormap not being even

[Wumpf]

What

Pointed out in #3373 that our grey colormap looked broken in the preview. When diving into it I got at first a scare about our color space pipeline being broken, but eventually I concluded that it was only the gray colormap which was broken.

Grey before/after with reference gradient from gimp:

It's not perfectly equal to the reference, but then again the test wasn't executed with maximum vigor either. Some slight differences are to be expected due to the amount of conversions we need to go through to arrive at the final image.

(unchanged) turbo colormap gradient compared with the render from Matplot webpage (a bit sloppy but shows that we're definitely not some wild gamma converstion away from it!):

This affects all usages of the greyscale colormap.

Checklist

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• I've included a screenshot or gif (if applicable)
• I have tested demo.rerun.io (if applicable)

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[emilk]

I'm 100% sure I made this fix before… maybe it was to color.rs (which is correct!)