perform GrahamScanConvexHull2D around an arbitrary oriented 2D plane in 3D, to fix some convex hull face merging problems

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)

Demos/OpenGL/DemoApplication.cpp

@@ -33,7 +33,7 @@ subject to the following restrictions:
 #include "LinearMath/btSerializer.h"
 #include "GLDebugFont.h"
 
-static bool use6Dof = false;
+
 extern bool gDisableDeactivation;
 int numObjects = 0;
 const int maxNumObjects = 16384;
@@ -535,7 +535,7 @@ void	DemoApplication::setShootBoxShape ()
 	if (!m_shootBoxShape)
 	{
 		btBoxShape* box = new btBoxShape(btVector3(0.5,0.5,0.5));
-        box->initializePolyhedralFeatures();
+      //  box->initializePolyhedralFeatures();
 		m_shootBoxShape = box;
 	}
 }
@@ -776,7 +776,7 @@ void DemoApplication::mouseFunc(int button, int state, int x, int y)
 								
 
 
-								if (use6Dof)
+								if ((m_modifierKeys& BT_ACTIVE_SHIFT)==0)
 								{
 									btTransform tr;
 									tr.setIdentity();
@@ -822,8 +822,7 @@ void DemoApplication::mouseFunc(int button, int state, int x, int y)
 									
 
 								}
-								use6Dof = !use6Dof;
-
+					
 								//save mouse position for dragging
 								gOldPickingPos = rayTo;
 								gHitPos = pickPos;