With exception of Sequencer, everywhere else in Blender it is assumed
that float images are in linear color space.
Movie files with more than 8 bit/channel precision are now loaded into
float images (since 39c4c7cf3f), but the "load and use them as an
image" code path was not making sure that they are converted to linear
color space.
Pull Request: https://projects.blender.org/blender/blender/pulls/131141