add compound shape support to BulletXmlWorldImporter and fix some compile issue under Debian (hopefully) object picking change in the demos: create a ball-socket picking constraint when holding shift while mouse dragging, otherwise a fixed (6dof) constraint add assert in constraint solver, when both objects have their inertia tensor rows set to zero btPolyhedralContactClipping: add edge-edge contact point in findSeparatingAxis (similar to the default GJK case)