very simple pure CPU raymarching demo

[ketmar via Digitalmars-d-announce]

ok, just4fun, mulththreaded renderer[1]. set ThreadCount to 
number of your CPU cores to get some speedup.

note: this is not how `std.concurrency` should be used! please, 
don't do wroker queues as i did!

[1] http://ketmar.no-ip.org/dmd/zrm3_adam_trd_x4.d