The current clay strips brush has a huge dynamic range which is nice for
certain effects, but in some pen tablets it is quite hard to control on
the higher values and it ends up producing unwanted artifacts. This
changes the input curves a bit in an attempt to make it more
controllable.

After adding a brush management system I will expose all these values to
be configurable per brush (the round version of clay strips is a little
bit better with different curves, but that is something that we can't
support now), but for now this default are probably safer.