Initial implementation of bg sky sphere.
Working background sky sphere.
Implemented do_load_bg_modelview() method that extracts rotation component from
current modelview matrix.

This is important for insert_background() that must inherit rotation component
from ancestor transforms + user rotation.

The solution is not perfect since it is not imune to shear transformation component.
Moved do_load_bg_modelview() and do_insert_background() to boxfish_vrml_bg.cpp
Created todo list for the project using ToDoList from CodeProject.
http://abstractspoon.pbworks.com/w/page/1262208/FrontPage
Make background to be unaffected from polygon and shading mode.
Remove check for maximized bounding volume in scene bounds calculation.

This check is not required since maximized property does now have a different meaning.
Refactored sky_extrusion_generator into more generic class capable to create
both sky and ground (sub)spheres.
Minimal working background support. Still missing ground and panoramic parts.
Implemented ground part of the background node.
Make background colors liniarly interpolate in the middle of zones defined by sky/ground angles.
Switch from GL_CLAMP to GL_CLAMP_TO_EDGE to elimniate edge artefacts when drawing texture.
Implement background node panorama cube.
Fix. Background panorama sides that have no image defined inherited image from
previously drawn sides (that have image defined). This led to full panorama
when it was not supposed to be full.