Fix white background flashing up when using Invert Lightness/Luma
requested to merge davidhurka/okular:fix-white-background-with-invert-lightness-luma into release/21.04
These color modes make white background black, but PagePainter wasn’t updated to draw black background when pixmaps are not yet available.
Low-risk bugfix with potentially high gain to flicker-sensitive people, so targeting release branch.