Fix transparency checkers looked white on HDR display

On HDR displays before this commit transparency checkers
looked white because it was just a SDR image showed in
full HDR glory. Hence both light and darker light grey
checkers looked just bright white, no different from each
other, even though in fact their colors were different.
This commit fixes this behaviour by converting the colors
properly from SDR/sRGB to Rec2020-pq (or other display format)
as it should be done.

BUG:407238
CCBUG:406698