Blender Git Loki

Main solver step for generating a divergence-free hair velocity field
on the grid.

This uses the Eigen conjugate-gradient solver to solve the implicit
Poisson equation for the pressure Laplacian:

div(grad(p)) = div(v)

As described in "Detail Preserving Continuum Simulation of Straight Hair"
(McAdams, Selle, 2009).

Conflicts:
source/blender/physics/intern/BPH_mass_spring.cpp