[BUGFIX] getPageOverlay_preProcess respect changes done in hooks

Due to the implementation of getPagesOverlay the individual page array
is passed to the getPageOverlay_preProcess hook without
being able to use changes done in user function
to the page array.
Now the page array can be modified again.
The change was introduced by adding the feature
PageRepository::getPagesOverlay() in http://review.typo3.org/36054

Change-Id: If4a07b232b3e34eb146f8ea06abe32efc7b72fdb
Releases: master, 7.6
Resolves: #76031
Relates: #64258
Reviewed-on: https://review.typo3.org/47996


Reviewed-by: Nicole Cordes <typo3@cordes.co>
Tested-by: Nicole Cordes <typo3@cordes.co>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>