[BUGFIX] Render metatags on uncached pages

Adding metatags on uncached pages using the pagerenderer didn't work
because the metatag array was rendered prior to adding any
entries with the 'render-preProcess' hook.

Resolves: #79162
Releases: master, 7.6
Change-Id: I2142a5c91fcb0840993d9235caa1e883bbb0c2b4
Reviewed-on: https://review.typo3.org/51143


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Frans Saris <franssaris@gmail.com>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Frans Saris <franssaris@gmail.com>
Reviewed-by: Stephan Großberndt <stephan@grossberndt.de>
Reviewed-by: Daniel Goerz <ervaude@gmail.com>
Reviewed-by: Andreas Fernandez <typo3@scripting-base.de>
Reviewed-by: Joerg Boesche <typo3@joergboesche.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>