Enhance LogCameraTransform to support additional parametric styles

[doug-walker]

The existing LogCameraTransform is able to provide a very compact representation for many of the "pseudo-log" transfer functions that are used in current cameras (LogCv3, Log3G10, SLog3, VLog, etc.) and ACEScct. These are basically a LogAffineTransform that switches to a straight line in the shadow region. However, there are a few camera color spaces that don't fit this model because a given transfer function has something more complicated than a straight line continuing into the shadow and negative values. It would be nice to extend LogCameraTransform to handle these as well.

Details to follow ...

[doug-walker]

After discussion, because these alternative log camera formulas are somewhat unusual, we decided to implement this as new FixedFunctions rather than extend LogCameraTransform. Closing this as fixed via PR #2029.