Use face index for soft contact, various other changes.

Remove btVertexBufferDescriptor, minor reformatting in
btSoftBodyHelpers.h/cpp.

Remove appendLink()/appendFace() using 'int model'.
Remove m_restLengthScale and unused fDrawFlags; simplify links.
Remove m_normal and m_area from face, remove cluster demos.

Also move btSoftBody::refine() to another class.