To align better with the pixel and reduce the samples needed.
The paper was using gamma because the jacobian |d_gamma/d_h| approaches
infinity at the boundaries, but it seems that clamping at 0.999 is
enough for numerical stability.
In practice I did not notice a change in the noise level, but it
simplifies the range computation and renders faster due to reduced
sample amount.
Co-authored-by: Olivier Maury <omaury@meta.com>
Ref: !129616
Pull Request: https://projects.blender.org/blender/blender/pulls/134130
b76fbb285e