Some performance numbers

Std scene, 640x480

Single threaded

real 0m45.319s user 1m11.707s sys 0m0.728s

real 0m45.428s user 1m12.641s sys 0m0.689s

real 0m44.951s user 1m11.773s sys 0m0.687s

One pixel per worker

real 0m19.696s user 2m11.190s sys 0m1.676s

real 0m19.948s user 2m15.547s sys 0m1.729s

One line per worker

real 0m19.516s user 2m3.791s sys 0m2.076s

real 0m19.387s user 2m1.752s sys 0m1.910s

real 0m18.921s user 1m59.795s sys 0m1.933s

New big: 17m14.517650246s