effects/zoom: Render scene in an offscreen texture

This fixes integration of the zoom effect with the blur effect and qtquick
effects such as overview or window view.

The main con of the proposed solution is that offscreen rendering can be
a bit overkill. But on the other hand, it's the most robust way to
ensure that fullscreen effects integrate correctly with the zoom effect.
For example, without it, the quick scene effect would need to figure out
what screen views to paint for the given screen. There are also more
finicky cases, e.g. the screen transform effect.

BUG: 447002
BUG: 447670
BUG: 453467
Tested-by: Merge Service <!2451>
Part-of: <!2451>