Merge pull request #1091 from YunfeiBai/master

Add APIs to reset object mass, lateral friction coefficient, and to get user constraint id.

src/BulletDynamics/Featherstone/btMultiBody.h

@@ -140,6 +140,11 @@ public:
 		return m_baseCollider;
 	}
 
+	btMultiBodyLinkCollider* getLinkCollider(int index)
+	{
+		return m_colliders[index];
+	}
+
     //
     // get parent
     // input: link num from 0 to num_links-1