Prior to using OIIO for image loading, DDS files were hard-coded to
always treat files with BC5/ATI2 compression as normal maps[1].
This basically means that the B channel would be reconstructed from the
R,G channels in a particular way. There is a non-standard header flag
(coming from tools like NVTT and various others) that can also be used
to indicate if such processing should take place, and OIIO understands
that flag and acts appropriately. However, not all files have that flag
set.
This patch reverts to the hard-coded behavior to match prior versions.
If the user has explicitly set the OIIO environment variable to say
otherwise, we will respect that setting instead.
[1] https://projects.blender.org/blender/blender/src/branch/blender-v3.5-release/source/blender/imbuf/intern/dds/DirectDrawSurface.cpp#L870
Pull Request: https://projects.blender.org/blender/blender/pulls/109541
5c37e2123e