Spline interpolation for the new hair system

While the new hair system will most likely have child splines like the current one, I think it's important to improve on the current system.

• Currently, the interpolation is spatial, which makes thin geometry (such as the ears of an animal) very hard to groom. Since the new system already requires UVs, compared to the current one, they should be used for interpolation.
• Interpolation is currently only linear, but smoother options would be nice to have.