Use Box2D's callbacks for destruction of physics objects and wait until the end of the timestep before destroying them.

Merge of box2d-id2 from love-experiments

src/modules/physics/box2d/Fixture.h

@@ -62,8 +62,8 @@ namespace box2d
 	protected:
 
 		Body * body;
-		b2Fixture * fixture;
 		fixtureudata * data;
+		b2Fixture * fixture;
 
 	public:
 
@@ -90,6 +90,11 @@ namespace box2d
 		**/
 		Shape * getShape() const;
 
+		/**
+		* Returns true if the fixture is active in a Box2D world.
+		**/
+		bool isValid() const;
+
 		/**
 		* Checks whether this Fixture acts as a sensor.
 		* @return True if sensor, false otherwise.
@@ -206,6 +211,11 @@ namespace box2d
 		* This operation may be expensive.
 		**/
 		int getMassData(lua_State * L) const;
+
+		/**
+		* Destroys this fixture.
+		**/
+		void destroy(bool implicit = false);
 	};
 
 } // box2d