Backward tracing of photons. mc_backward takes either zero, two or four coordinates:
mc_backward [ix_start iy_start] [ix_end iy_end]
where ix_start, iy_start is the index of the sample pixel to be calculated
or the pixel area from ix_start to ix_end and iy_start to iy_end.
All x-indices must be in the range of 0 … (Nx-1) and y-indices the range of 0 … (Ny-1).
If no coordinates are specified, all sample pixels will be calculated.
mc_backward computes radiances and downward diffuse irradiances. If
a different quantity is required, please use mc_backward_output.