Extends the changes started in f31c3f8114616bb to completely separate
much of the DRW curves code from the particle hair drawing. In the short
term this increases duplication, but the idea is to simplify development
by making it easier to do larger changes to the new code, and the new
system will replace the particle hair at some point.
After this, only the shaders themselves are shared.
Differential Revision: https://developer.blender.org/D14699
