Add application attribute to disable highDPI scaling

Main use case is for KDE Plasma, which needs to know about real
window system coordinates. Can also be used by applications that
do all the scaling manually, and that do not want to have Qt
scale the coordinate systems and painters.

Change-Id: Ic595131c5e2fb4c0af95e9b5bd3e40beee86760b
Reviewed-by: Morten Johan Sørvig <morten.sorvig@theqtcompany.com>

src/corelib/global/qnamespace.h

@@ -496,6 +496,7 @@ public:
         AA_UseSoftwareOpenGL = 17,
         AA_ShareOpenGLContexts = 18,
         AA_SetPalette = 19,
+        AA_NoHighDpiScaling = 20,
 
         // Add new attributes before this line
         AA_AttributeCount

src/corelib/global/qnamespace.qdoc

@@ -197,6 +197,11 @@
     \value AA_SetPalette Indicates whether a palette was explicitly set on the
            QApplication/QGuiApplication. This value has been added in Qt 5.5.
 
+    \value AA_NoHighDpiScaling Disables all high-DPI scaling in Qt, exposing window
+           system coordinates. Note that the window system may do its own scaling,
+           so this does not guarantee that QPaintDevice::devicePixelRatio() will
+           be equal to 1. This value has been added in Qt 5.6.
+
     The following values are obsolete:
 
     \value AA_ImmediateWidgetCreation This attribute is no longer fully

src/gui/kernel/qhighdpiscaling.cpp

@@ -142,6 +142,11 @@ QDpi QHighDpiScaling::m_logicalDpi; // The scaled logical DPI of the primary scr
 */
 void QHighDpiScaling::initHighDpiScaling()
 {
+    if (QCoreApplication::testAttribute(Qt::AA_NoHighDpiScaling)) {
+        m_factor = 1;
+        m_active = false;
+        return;
+    }
     m_factor = initialScaleFactor();
     bool usePlatformPluginPixelDensity = qEnvironmentVariableIsSet(autoScreenEnvVar)
                                          || qgetenv(legacyDevicePixelEnvVar).toLower() == "auto";
@@ -157,6 +162,9 @@ void QHighDpiScaling::initHighDpiScaling()
 
 void QHighDpiScaling::updateHighDpiScaling()
 {
+    if (QCoreApplication::testAttribute(Qt::AA_NoHighDpiScaling))
+        return;
+
     if (m_usePixelDensity && !m_pixelDensityScalingActive) {
         Q_FOREACH (QScreen *screen, QGuiApplication::screens()) {
             if (!qFuzzyCompare(screenSubfactor(screen->handle()), qreal(1))) {