Use t_object (t_element) for lights and cameras

Use t_object (t_element) for lights and cameras

This is because manipulation (translation and rotation) has to be
implemented not just for objects but for lights and cameras also.
It would be better to create a separate union from t_object, but I
cannot think of a way that would not be quite difficult to use.

The typedef t_element can be used for generic element, whereas t_object
should be used for objects (things casting shadow) only.

The vectors for lights and cameras was also changed to vectors of
t_element so that it is easy to pass it as a t_element pointer to change
it in place.