Option to compute an export variance
Added an option ("-log_variance") to compute per pixel, per "wavelength" variance. The result is saved along with the output image ("name_of_output" + "_variance") using the same extension. The number of samples, the mean variance (over the image) per chanel and the rendering time is added as comments in the image header.
Tested with offline and interactive, in RGB an spectral.
Edited by Alban Fichet