App: Use the same high dpi scale factor rounding as Qt 5 does

After brief usage of a Qt 6 build of Qt Creator under non-rounded
scaling factors such as 150% or 175%, all kinds of rendering artifacts
are noticeable.

Proper support of these scaling factors requires fixes in several
places. This change is the quickest alternative fix/workaround: restore
the behavior of Qt 5.

Fixes: QDS-4739
Fixes: QTCREATORBUG-25862
Change-Id: Ia8bfda088ee20ebbe08cb4f79325d5a51ef7124b
Reviewed-by: Thomas Hartmann <thomas.hartmann@qt.io>

src/app/main.cpp

@@ -476,6 +476,8 @@ int main(int argc, char **argv)
         QCoreApplication::setAttribute(Qt::AA_UseOpenGLES);
 #else
     qputenv("QSG_RHI_BACKEND", "opengl");
+    QGuiApplication::setHighDpiScaleFactorRoundingPolicy(
+                Qt::HighDpiScaleFactorRoundingPolicy::Round);
 #endif
 
     if (qEnvironmentVariableIsSet("QTCREATOR_DISABLE_NATIVE_MENUBAR")