kcm/users: fix rectangle grabbing

the earlier attempt at fixing this unfortunately only broke things
differently because grabbing the entire component means we end up with
spacing around the colored rectangle (the square artifacts described in
https://bugs.kde.org/show_bug.cgi?id=479929). instead alias the
rectangles to the outside and restore the grabToImage call to operate on
the colorRectangles rather than the entire button

BUG: 479454
(cherry picked from commit 7eddcd47)