avfilter: Add blue and violet noise generation filters

For the blue and violet noise, I took the pink and brown noise
respectively and subtracted the offsets instead of adding them. When I
eyeball the frequency spectrum of the resulting outputs it looks correct
to me, i.e. the blue graph appears to be a mirror image of the pink, and
the same can be said of the violet and the brown. I did not do anything
else to confirm the correctness.

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