[BUGFIX] Provide correct FormEngine instance to userFunc

Use the FormEngine while calling the UserFunction inside the UserElement
as this was expected before splitting the elements out of FormEngine
class.

Resolves: #62060
Releases: master
Change-Id: I249b6a50cb95ee69c34b9d1d6a24ac19934e66a1
Reviewed-on: http://review.typo3.org/33174
Reviewed-by: Markus Klein <klein.t3@reelworx.at>
Tested-by: Markus Klein <klein.t3@reelworx.at>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>