Add gravity, starting and stopping simulation

source/engine/2dphysics.h

@@ -101,6 +101,7 @@ void phys2d_update(float deltaT);
 void phys2d_apply();
 
 void phys2d_add_handler_type(int cmd, struct gameobject *go,  s7_pointer cb);
+void phys2d_set_gravity(float x, float y);
 
 void shape_gui(struct phys2d_shape *shape);