re-organized the export list and exported some needed functions

re-organized the export list and exported some needed functions
created a raytraced_pixel_list_simple() - not concurrent
renamed raytraced_pixel_list() raytraced_pixel_list_concurrent()
introduced a worker process for concurrent tracing
stated an assumption of the concurrent system
implemented a worker process that passes the work to trace_ray_from_pixel()
removed the concurrency variables and concerns from trace_ray_from_pixel()
removed the go() function to make way for various invocation styled functions
added a standalone() invocation function
implemented standalone_concurrent() and standalone_simple() functions
implemented go_concurrent() and go_simple() functions as well
removed warnings related to unused variables and functions

git-svn-id: http://svn.plouj.com/repos/eraytracer@42 b8c14938-b733-429d-887c-d66ed1f8cd15