Set AA_EnableHighDpiScaling attribute early

Qt docs: This attribute must be set before QGuiApplication is 
constructed.

src/qt/bitcoin.cpp

@@ -436,16 +436,17 @@ int GuiMain(int argc, char* argv[])
     Q_INIT_RESOURCE(bitcoin);
     Q_INIT_RESOURCE(bitcoin_locale);
 
-    BitcoinApplication app(*node, argc, argv);
     // Generate high-dpi pixmaps
     QApplication::setAttribute(Qt::AA_UseHighDpiPixmaps);
 #if QT_VERSION >= 0x050600
-    QGuiApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
+    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
 #endif
 #ifdef Q_OS_MAC
     QApplication::setAttribute(Qt::AA_DontShowIconsInMenus);
 #endif
 
+    BitcoinApplication app(*node, argc, argv);
+
     // Register meta types used for QMetaObject::invokeMethod
     qRegisterMetaType< bool* >();
     //   Need to pass name here as CAmount is a typedef (see http://qt-project.org/doc/qt-5/qmetatype.html#qRegisterMetaType)