[BUGFIX] Regenerate missing RTE magic image on rendering

Problem: Magic images inserted in RTE content may be missing,
for instance after flushing the _processed_ directory.
Solution: Regenerate any missing magic image when rendering the content
element.

Revolves: #56147
Releases: 6.2
Change-Id: Ifa156791bcd1c446c7ae61888465bb680abdf36d
Reviewed-on: https://review.typo3.org/27884
Reviewed-by: Frans Saris
Tested-by: Frans Saris
Reviewed-by: Fabien Udriot
Tested-by: Fabien Udriot
Reviewed-by: Wouter Wolters
Reviewed-by: Stanislas Rolland
Tested-by: Stanislas Rolland