Add physics time scale in VR mode
Fix compound collision margin in physics server
Enable visuals for rigid body (non-multi-body) URDF/SDF loading in physics server.
add rolling/spinning friction to cube, remove it from plane/samurai.urdf
URDF2Bullet: support joint limits for revolute and prismatic, only if defined (if upper < lower, disable limit)
add some profiling markers to improve performance