[TASK] Use constants for 'Image.CropScaleMask' and 'Image.CropScaleMask'

Constants ProcessedFile::CONTEXT_IMAGECROPSCALEMASK and
ProcessedFile::CONTEXT_IMAGEPREVIEW are now used instead of strings.

Resolves: #81790
Releases: master
Change-Id: I3f8aa0dcbe4cb292f8fe8c6993ecf025c786d70b
Reviewed-on: https://review.typo3.org/53400


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>