game: remove scene locking

all these locking/pending/etc make things
more complicated, one example is adding new
object with joints in 'update' method, with
current model it's not possible since 'update'
is 'locked' and all objects added inside 'update'
will be registered only after all 'update's are
complete, i.e. we can't do this:

SceneObjectPtr obj = ...;

scene()->addObject(obj);

b2JointDef jointDef;

jointDef.xxx = obj->body(); // body is NULL here

scene()->addJoint(jointDef);

now we just remove all that delayed stuff and
register things immediately, so the code above
is now possible. Of course, we have to be
more careful now and don't do things like these
inside 'update':

parent()->removeFromParent();

scene()->xxx(); // scene() will return NULL here

but these are pretty obvious, so I guess removing
scene locking does make sense