bounding_sphere can now be maximized without loss of center and radius information.

This change is dictated by moving background rendering into node domain (not yet checked in)
out of metatype domain. The above change is required by VRML spec that requires
accumulated ancestor transform rotations to apply on bg node.

Bounding sphere of the background must be 'maximized' but that would make the entire scene
bounding volume maximized (having huge radius) which will affect the examination view.

The change introduces 'maximized_' flag in the bounding_sphere. Extending the sphere now
works as usually + maximize flag is inherited. Thus bounding volume of the scene with
bg or other maximized node will have center and radius dictated by inner geometries
and the maximized flag set. The former will correctly present the scene size, while the
later will make a correct frustum intersection.