Change coefficients for the rangecompress/expand transformation.

Modifications to range compression/expansion.

The coefficients we use to do range compression turn out not to work well in practice, nor does it match the old tool we use at SPI that oiiotool is replacing.  Actually, I was unintentionally given sub-optimal values when I first implemented it in OIIO, when better ones were being used elsewhere but we didn't realize there were two sets floating around, oops.

An intersting consequence is that the old coefficients did not alter values <= 1, so a LDR image would by definition not need the transformation applied.  The better coefficients, unfortunately, do compress the range even for values < 1, which is unfortunate, but at the end of the day the new coefficients do a better job of the intended purpose.

This patch:

* Alters the coefficients used for the logarithmic<->linear transformation used by IBA::rangecompress / rangeexpand.  These work better in practice.

* The docs have been altered to no longer say explicitly that values < 1 are unchanged, nor that it's a no-op for LDR image formats.

* Remove various code shortcuts that avoided the transformation for LDR images or values < 1.  Gotta do it all the time for results to match correctly.

* Changes the default for rangecompress/expand to transform each color channel independently, rather than use the luma.  The option is still there, we're just changing the default to the one that years of experience at SPI was preferred.  This applies both to the IBA functions themselves, as well as the commands in oiiotool.

Please note that this also applies to the behavior of maketx when the --hicomp option.