[PATCH] QColorTransferGeneric: fix the BT.2100 PQ EOTF

[PATCH] QColorTransferGeneric: fix the BT.2100 PQ EOTF

The PQ EOTF formula for BT.2100 [1][2] was incorrect. Fix it; while at
it, rename the variables to match the symbols used in the original
formula.

The inverse EOTF was correct, but also rename the variables there (for
the same reason).

[1] https://www.itu.int/rec/R-REC-BT.2100-2-201807-I/en
[2] https://en.wikipedia.org/wiki/Perceptual_quantizer#Technical_details

Change-Id: I6ce3a609824bee82053a16b3ff3cfc7cb396ce8f
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
(cherry picked from commit a7ff4679facb9a44dff8b63a7e461ababa6aedfb)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 35c18eba64b6220e36ea14d32b911342e9d0921c)
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Gbp-Pq: Name upstream_QColorTransferGeneric_fix_BT2100PQEOTF.patch