Delegate GUI scaling work to Qt

Set Qt::AA_EnableHighDpiScaling application attribute when Qt 5.14.x or
newer is used. This fixes a lot of scaling issues on HiDPI displays.

Unfortunately, this flag must be set only before QApllication object
creation, so the only one place where it can be done is main().

src/app/main.cpp

@@ -133,6 +133,12 @@ int main(int argc, char *argv[])
     // We must save it here because QApplication constructor may change it
     bool isOneArg = (argc == 2);
 
+#if !defined(DISABLE_GUI) && (QT_VERSION >= QT_VERSION_CHECK(5, 14, 0))
+    // Attribute Qt::AA_EnableHighDpiScaling must be set before QCoreApplication is created
+    if (qgetenv("QT_ENABLE_HIGHDPI_SCALING").isEmpty() && qgetenv("QT_AUTO_SCREEN_SCALE_FACTOR").isEmpty())
+        Application::setAttribute(Qt::AA_EnableHighDpiScaling, true);
+#endif
+
     try {
         // Create Application
         const QString appId = QLatin1String("qBittorrent-") + Utils::Misc::getUserIDString();

src/gui/utils.cpp

@@ -62,6 +62,10 @@ void Utils::Gui::resize(QWidget *widget, const QSize &newSize)
 
 qreal Utils::Gui::screenScalingFactor(const QWidget *widget)
 {
+#if (QT_VERSION >= QT_VERSION_CHECK(5, 14, 0))
+    Q_UNUSED(widget);
+    return 1;
+#else
     if (!widget)
         return 1;
 
@@ -77,6 +81,7 @@ qreal Utils::Gui::screenScalingFactor(const QWidget *widget)
 #else
     return widget->devicePixelRatioF();
 #endif // Q_OS_WIN
+#endif // QT_VERSION
 }
 
 QPixmap Utils::Gui::scaledPixmap(const QIcon &icon, const QWidget *widget, const int height)