made nearest_object_intersecting_ray agnostic to the object types

made nearest_object_intersecting_ray agnostic to the object types
made nearest_object_intersecting_ray actually properly sort objects by distance from ray origin
added a general ray_object_intersect function that will call appropriate intersection functions based on object type
made sure ray_sphere_intersect only returns positive (in front of ray) values
fixed return value of ray_sphere_intersect (from none to infinity)
made the default scene mor complex
updated the scene_test
added a nice distance based test to nearest_object_intersecting_ray_test

git-svn-id: http://svn.plouj.com/repos/eraytracer@15 b8c14938-b733-429d-887c-d66ed1f8cd15