"static_cast from 'CGContextRef' (aka 'CGContext *') prvalue to 'CGContextRef'
(aka 'CGContext *') prvalue is redundant" with
--with-macosx-version-min-required=10.14 at least.  (Not exactly sure why this
warning only starts to appear now.  Presumably something changed in the system
headers.  Lets see whether older baseline builds are happy with the dropped
casts, too.)

Change-Id: Id52d1891e5e33f7c37d095440339980b4000ca4e
Reviewed-on: https://gerrit.libreoffice.org/61353
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>