Add m_combinedFriction2 to btManifoldPoint.

[kermado]

You have m_lateralFrictionDir2 and m_contactMotion2, but not m_combinedFriction2. ODE supports this through the dContactMu2 mode flag and dSurfaceParameters::mu2.

[erwincoumans]

Thanks for the feedback. Yes, I'll look into adding this.