vcl-osx fix: 'NSEvent' may not respond to '-magnification'

(error: invalid operands of types 'float' and 'objc_object*' to binary 'operator+')

Change-Id: Iade7acde31fad21d7dde1a8c20c8900d9cbbd446
Reviewed-on: https://gerrit.libreoffice.org/8792
Tested-by: Norbert Thiebaud <nthiebaud@gmail.com>
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>