Updates the `CurveMapping` `CURVE_PRESET_SMOOTH` curve to more closely
approximate the smoothstep function (y = 3x^2 - 2x^3). This has the
implication of changing what happens when the curve is reset, but does
not change any existing curves already stored in blendfiles.
Notably, this change primarily affects three operators:
* brush.curve_preset (Mesh Sculpting)
* brush.sculpt_curves_falloff_preset (Curves Sculpting)
* render.shutter_curve_preset (Motion Blur)
Pull Request: https://projects.blender.org/blender/blender/pulls/146710
9b5f23c7a3