Object::setClean() now computes absolute transformation while traversing
through object parents and passes it as parameter to clean(), which is
now virtual a meant to be reimplemented instead of setClean().
Updated and greatly improved unit test.
It was overengineered and unnecessarily complicated. Now the camera is
specified only in Scene::draw(), which eliminates all the needs for
recalculating absolute object transformations on each camera
transformation change. Absolute object transformation is now computed
relative to root object or relative to camera object passed as
parameter. Because of that it is now also possible to draw the scene
using multiple cameras at once.