the multiframe loaders sets the color modifier to NULL but doesn't
restore it before returning. this results in a imlib2 developer warning
if you try to change brightness/contrast on a multiframe image (which
doesn't have alpha).
ensure that the color modifier is restored before returning under all
paths.
Reported-by: Madhu
Reviewed-on: https://codeberg.org/nsxiv/nsxiv/pulls/440
Reviewed-by: eylles <eylles@noreply.codeberg.org>