QPdf::addImage(): avoid a QImage detach when it's in an acceptable Format

Don't detach QImage, when it's in Format_Mono or Format_ARG32.
Use QImage::constScanLine() instead of QImage::scanLine().

Change-Id: I30fcafb576aea3189637a40fd75f77c70017ba46
Reviewed-by: John Layt <jlayt@kde.org>
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>
(cherry picked from qtbase/1a32cc0ae42a4a49f2d45b8ad8251ec40a30bf76)
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>