This adds a new `Curve Falloff` popover to the comb brush tool settings.
The curve control allows changing the brush weight along the curve to
e.g. affect the tip more than the root. This is a relative way to get
something like stiffness for short hair.
This functionality could potentially be added to some other brushes,
but the comb brush is the most important one, so that is added first.
I did add the buttons add the buttons to choose a curve map preset.
However, I did not add the preset dropdown, because that just adds
some unnecessary complexity in the code now and is redundant.
Pull Request #104589
ddc6815b1a