Improved URDF support for btMultiBody and separate graphics/collision/inertial frames and shapes

Fix WinXP GetTickCount64 with a typedef
Expose debug drawing mode/flags in UI (hot keys A,D,L,W for now, buttons later)
GLInstancingRenderer: tweak near/far planes to allow closer approach of camera
btDiscreteDynamicsWorld: enable debug drawing for btGeneric6DofSpring2Constraint
btMultiBodyDynamicsWorld: enable basic debug drawing for btMultiBody
btMultibody: allow center-of-mass shift for prismatic and fixed constraint

src/LinearMath/btQuickprof.cpp

@@ -43,6 +43,11 @@ static btClock gProfileClock;
 	#include <Xtl.h>
 #else //_XBOX
 	#include <windows.h>
+
+#if WINVER < 0x0600
+ULONGLONG GetTickCount64() { return GetTickCount(); }
+#endif
+
 #endif //_XBOX
 
 #include <time.h>