This is an alternative method to the current fixed function with a
clump factor and "shape" parameter. This function is quite limited and
does not give the desired result in many cases (e.g. long, parallel
rasta strands are problematic). So rather than trying to add more
parameters there is now a fully user-defined optional curve for setting
the tapering shape.
This contains a few pieces of code for a future "modifier" system that
would allow more flexible combination of effects. Eventually a node
system is the way to go, but the current code makes that impossible.
