For the c++ api dox this expands to "\c nullptr" (the
\c directive indicates a code literal value), and for sipify/Python
it expands to ``None`` (`` is sphinx annotation for literal values)
Makes for nicer dox for both c++ and Python!
This adds new options for user to choose how the terrain should be rendered:
- shading disabled - color of terrain is determined only from map texture
- shading enabled - color of terrain is determined using Phong's shading model,
taking into account map texture, terrain normal vector, scene light(s) and
terrain material's ambient+specular colors and shininess
(configuration of lights is not there yet - now we always have a single point
light with white color at the middle of the scene at 1km above the zero elevation)
