Fix infinite loop when a clone layer is connected to a group with clones

When a clone layer tries to prepare the source region on the source, it used to cause notification on the clones inside the source. These clones caused the source group to become dirty and trigger cyclic updates on the original clone.

The patch just disables the clone update notifications for "hidden area preparation" pass. Ideally, we should refactor these clone notifications to be included into the walkers themselves. Then they will have full context of the update and may be limited to not go farther than the requested clone source.

For now, "hidden area preparation" pass just doesn't trigger any clone updates. Anyway, these clone updates will not be visible in the source clone, since they will be executed asynchronously, after the source clone has finished its rendering.

See this KA report: https://krita-artists.org/t/files-opening-very-slowly-or-not-at-all-after-updating-from-5-2-2-to-5-2-6/103786/