We want the default stroke width to be represented by a position
close to the center of the stroke width slider, with max value being
eight times the default value (four times for the blur tool) and min
value being 20% of the default value.
To do that slider value (which grows linearly) is raised
to power of 3 (2 for blur) when that value is > 1, and only then
that value becomes a multiplier for default stroke width.
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