warmstarting for contact points was broken, fix in btPersistentManifold
enable split impulse by default (at the cost of some performance)
add the option for zero-length friction (instead of recomputing friction directions using btPlaneSpace), use the solver mode flag SOLVER_ALLOW_ZERO_LENGTH_FRICTION_DIRECTIONS
precompute lateral friction directions (in btManifoldResult)
remove the mConstraintRow[3] from btManifoldPoint, it just took a lot of memory with no benefits: fixed it in btParallelConstraintSolver