Respect the custom paper size settings on Mac

When the user added a custom paper size then it would be silently
ignored when printing on Mac. This now ensures that it is respected when
appropriate.

Manual cherry-pick from Qt 5: 26a0a3ed85008c3a0edae125fca48c7c36a7437f

[ChangeLog][Platform Specific Changes][OS X][QtPrintSupport] Respect
the custom paper size settings when printing.

Task-number: QTBUG-34700
Change-Id: If2ff9b704a636c94510f826d300e53af99c8e7ab
Reviewed-by: John Layt <jlayt@kde.org>