Move QT_FONT_DPI to cross-platform code

This makes it possible to test the effects of setting
Qt::AA_HighDpiScaling/QT_AUTO_SCREEN_SCALE_FACTOR, with different DPI
values on all platforms.

This also makes it possible to access the actual DPI values reported
by the OS/WS via the QPlatformScreen API.

A drawback is that there is no single place to check the environment
variable; currently done in three places. This may be
further simplified later on.

Done-with: Friedemann Kleint <Friedemann.Kleint@qt.io>
Task-number: QTBUG-53022
Change-Id: Idd6463219d3ae58fe0ab72c17686cce2eb9dbadd
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/gui/kernel/qhighdpiscaling.cpp

@@ -276,7 +276,7 @@ qreal QHighDpiScaling::rawScaleFactor(const QPlatformScreen *screen)
         qreal platformPhysicalDpi = screen->screen()->physicalDotsPerInch();
         factor = qreal(platformPhysicalDpi) / qreal(platformBaseDpi.first);
     } else {
-        QDpi platformLogicalDpi = screen->logicalDpi();
+        const QDpi platformLogicalDpi = QPlatformScreen::overrideDpi(screen->logicalDpi());
         factor = qreal(platformLogicalDpi.first) / qreal(platformBaseDpi.first);
     }
 
@@ -629,7 +629,7 @@ QDpi QHighDpiScaling::logicalDpi(const QScreen *screen)
         return QDpi(96, 96);
 
     if (!m_usePixelDensity)
-        return screen->handle()->logicalDpi();
+        return QPlatformScreen::overrideDpi(screen->handle()->logicalDpi());
 
     const qreal scaleFactor = rawScaleFactor(screen->handle());
     const qreal roundedScaleFactor = roundScaleFactor(scaleFactor);

src/gui/kernel/qplatformscreen.cpp

@@ -193,6 +193,14 @@ QDpi QPlatformScreen::logicalDpi() const
                 25.4 * s.height() / ps.height());
 }
 
+// Helper function for accessing the platform screen logical dpi
+// which accounts for QT_FONT_DPI.
+QPair<qreal, qreal> QPlatformScreen::overrideDpi(const QPair<qreal, qreal> &in)
+{
+    static const int overrideDpi = qEnvironmentVariableIntValue("QT_FONT_DPI");
+    return overrideDpi > 0 ?  QDpi(overrideDpi, overrideDpi) : in;
+}
+
 /*!
     Reimplement to return the base logical DPI for the platform. This
     DPI value should correspond to a standard-DPI (1x) display. The

src/gui/kernel/qplatformscreen.h

@@ -161,6 +161,8 @@ public:
     // The platform screen's geometry in device independent coordinates
     QRect deviceIndependentGeometry() const;
 
+    static QDpi overrideDpi(const QDpi &in);
+
 protected:
     void resizeMaximizedWindows();
 

src/gui/kernel/qscreen.cpp

@@ -84,8 +84,11 @@ void QScreenPrivate::setPlatformScreen(QPlatformScreen *screen)
     platformScreen->d_func()->screen = q;
     orientation = platformScreen->orientation();
     geometry = platformScreen->deviceIndependentGeometry();
-    availableGeometry = QHighDpi::fromNative(platformScreen->availableGeometry(), QHighDpiScaling::factor(platformScreen), geometry.topLeft());
-    logicalDpi = platformScreen->logicalDpi();
+    availableGeometry = QHighDpi::fromNative(platformScreen->availableGeometry(),
+                        QHighDpiScaling::factor(platformScreen), geometry.topLeft());
+
+    logicalDpi = QPlatformScreen::overrideDpi(platformScreen->logicalDpi());
+
     refreshRate = platformScreen->refreshRate();
     // safeguard ourselves against buggy platform behavior...
     if (refreshRate < 1.0)

src/gui/kernel/qwindowsysteminterface.cpp

@@ -857,8 +857,8 @@ void QWindowSystemInterface::handleScreenGeometryChange(QScreen *screen, const Q
 
 void QWindowSystemInterface::handleScreenLogicalDotsPerInchChange(QScreen *screen, qreal dpiX, qreal dpiY)
 {
-    QWindowSystemInterfacePrivate::ScreenLogicalDotsPerInchEvent *e =
-            new QWindowSystemInterfacePrivate::ScreenLogicalDotsPerInchEvent(screen, dpiX, dpiY); // ### tja
+    const QDpi effectiveDpi = QPlatformScreen::overrideDpi(QDpi{dpiX, dpiY});
+    auto e = new QWindowSystemInterfacePrivate::ScreenLogicalDotsPerInchEvent(screen, effectiveDpi.first, effectiveDpi.second);
     QWindowSystemInterfacePrivate::handleWindowSystemEvent(e);
 }
 

src/plugins/platforms/xcb/qxcbscreen.cpp

@@ -660,10 +660,6 @@ QImage::Format QXcbScreen::format() const
 
 int QXcbScreen::forcedDpi() const
 {
-    static const int overrideDpi = qEnvironmentVariableIntValue("QT_FONT_DPI");
-    if (overrideDpi)
-        return overrideDpi;
-
     const int forcedDpi = m_virtualDesktop->forcedDpi();
     if (forcedDpi > 0)
         return forcedDpi;