OpenEXR: As most channels are percieved in a logarithmic fashion by
human vision, pLinear default value has been changed to false.

This helps improve the apparent quality of b44 lossy compression.

In originally writing exroutpuut.cpp, LG misunderstood the pLinear flag
in the Imf::Channel constructor to be whether it was mathematically
linear, but really the flag is for whether it is approximately
perceptually linear.