Correct qt_defaultDpi X/Y with just a QCoreApplication

Makes the 96DPI attribute check avoid undefined behavior by using
QCoreApplication::instance() directly, instead of calling through
qApp, which performs an invalid cast to QGuiApplication.

Change-Id: Ib86e7d2461b462a2d623f1364414f7d4d2293f22
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>