[BUGFIX] CSC: file references not working with image compression

When using image compression with CSS styled content
all existing data in the $imgConf array is emptied.
If using a sys_file_reference, the file object
for resolving the file data does not consider
the option "treatIdAsReference" anymore as
it is unset as well.

How to reproduce: Add a file and select any image compression.
The wrong image is rendered, as the file UID is used
not the file reference UID.

If you have a sys_file UID 23, and a sys_file_reference ID 45
then the sys_file 45 is displayed instead of sys_file 23.

Releases: 6.2, 6.1
Resolves: #57346
Change-Id: Id503510b82df7bb4be9d236a1640322c92ffa5ec
Reviewed-on: https://review.typo3.org/28856
Reviewed-by: Tom Ruether
Tested-by: Tom Ruether
Reviewed-by: Wouter Wolters
Reviewed-by: Marc Bastian Heinrichs
Reviewed-by: Frans Saris
Tested-by: Frans Saris