This patch adds three new options for controlling curve radius:

• An option to use a CurveMap instead of the shape parameter to control the radius along the curve
• An option to use a Vertex Group as a multiplier of the curve radius
• An option to apply some random variation to the curve radius

There are two ToDos: The hashing that's currently used for the randomness isn't great,
and the options aren't supported in Eevee yet.