Improve damping formula in btRigidBody::applyDamping

Thanks to sparkprime, see http://code.google.com/p/bullet/issues/detail?id=74
Fixed btCompoundShape/btCapsuleShape/btGImpactShape for missing m_shapeType, and added an assert in DemoApplication::localCreateRigidBody for invalid shape types.

src/BulletCollision/CollisionShapes/btCompoundShape.cpp

@@ -24,6 +24,7 @@ m_collisionMargin(btScalar(0.)),
 m_localScaling(btScalar(1.),btScalar(1.),btScalar(1.)),
 m_dynamicAabbTree(0)
 {
+	m_shapeType = COMPOUND_SHAPE_PROXYTYPE;
 	void* mem = btAlignedAlloc(sizeof(btDbvt),16);
 	m_dynamicAabbTree = new(mem) btDbvt();
 	btAssert(mem==m_dynamicAabbTree);