backends: enable overlays by default for nested backends and for i915 and Intel XE drivers

Xaver Huglrequested to merge
work/zamundaaa/overlays-by-default into master

We haven't had any issues with overlays on Intel so far, so it should be safe to enable them in Plasma 6.7+.

Nothing changes for development versions, overlays are still enabled by default for everything except the proprietary Nvidia driver.

Merge request reports