When constant falloff is used, `BKE_brush_calc_curve_factors` doesn't do
any extra filtering of the given distances. The Plane brush previously
didn't filter the distances, leading to incorrect deformations when the
constant falloff was used.
To fix this, this commit makes a number of changes:
* `BKE_brush_calc_curve_factors` no longer sets the factor for an
element to 0 if it is outside of the provided distance. This is
replaced with an assert.
* The Plane brush and Cloth brush have an explicit
`filter_distances_with_radius` added.
Pull Request: https://projects.blender.org/blender/blender/pulls/139851
23951e1b12