This replaces per-image editor curve mapping which didn't behave properly
(it was possible to open the same image in two image editors and setup
different curves in this editors, but only last changed curve was applied
on image)
After discussion with Brecht decided to have something which works reliable
and predictable and ended up with adding RGB curves as a part of display
transform, which is applied before OCIO processor (to match old behavior).
Setting white/black values from image editor (Ctrl/Shift + LMB) would
affect on scene settings.
This could break compatibility, but there's no reliable way to convert
old semi-working settings into new one.
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