fix [#33781] Confusing units in displacement modifier parameters

16bit PNG images were not doing any conversions when saving linear float buffers, but would always load them as SRGB.
apply the same method used for 16bit images from floats as used in jpeg2000.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53807 954f8c5b-7b00-dc11-b283-0030488c597c