ScreenPaintData provides a way to transform the painted screen, e.g.
scale or translate. From API point of view, it's great. It allows
fullscreen effects to transform the workspace in various ways.

On the other hand, such effects end up fighting the default scene
painting algorithm. For example, just have a look at the slide effect!
With fullscreen effects, it's better to leave to them the decision how
the screen should be painted. For example, such approach is taken in
some wayland compositors, e.g. wayfire, and our qtquick effects already
operate in similar fashion.

Given that, strip the ScreenPaintData of all available transforms. The
main motivation behind this change is to improve encapsulation of item
painting code and simplify model-view-projection code in kwin. It will
also make the job of extracting item code for sharing purposes easier.