When EnumForms was used then the dmPaperSize was not always correct for
the custom paper sizes available on some printers. By using
DeviceCapabilities we can be sure that the information is correct in this
respect.
This also fixes respecting of the custom paper size if one is given and
there is no corresponding existing paper size for it.
Task-number: QTBUG-34276
Change-Id: I9924d5be8527027fc434261e37f6c7aae66210c3
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
3396ba5612