gifs with combined images not displayed correctly

It seems Qt does not support animated gifs with "combined images", i.e.
when each frame only contains changes to the previous frame and is put over the
previous one.
It animates the gif by showing only one frame each time and since part of
the frame is transparent to show the background frame, it does not give the
expected result.

(opening the gif with gimp shows "combine" next to each frame, except the
background one)

cf: https://bugs.kde.org/show_bug.cgi?id=225622