presentationwidget: Invalidate pixmaps on dpr change
Since the last rendered pixmap is usually reused, its device pixel ratio
can end up different from the devicePixelRatioF() of the widget which
is used for most size calculations which leads to an area that might be
unpainted causing visual artifacts.
@aacid @sune @nicolasfella @davidedmundson
The transition animations are still quite borked for me but I can reproduce that even under XWayland and get some glitches with integer scaling, too. Seems to be because it paints transparent (if I change the fill color to black in init transition it goes away) and then I get to see through the window into an area the compositor doesn’t repaint. Looks like I might also have to look into window formats since it sets WA_OpaquePaintEvent but right now QtWayland is always RGBA. Doesn’t explain XWayland, though.