[BUGFIX] Remove 1000 pixelx limitation for preview image

There is a limitation of 1000 pixels width / height when creating a
preview image, also known as ProcessedFile::CONTEXT_IMAGEPREVIEW

The limitation is artificial and subjective and should be removed letting
more freedom to the system, e.g for generating banner which are quite
large.

Change-Id: Id86c5491437a32b0011b4fe29129c3ee433712af
Resolves: #47846
Releases: 6.2, 6.1
Reviewed-on: https://review.typo3.org/20439
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Alexander Opitz
Tested-by: Alexander Opitz
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters