Use the "Round" HiDPI scaling policy (the default in Qt5) in Qt6

As of Qt 6.7.2 the rendering is completely broken and full of various render
artifacts when the default "PassThrough" policy is used and the user has
a fractional screen size like 125% set.

src/GUI/gui.cpp

@@ -1171,6 +1171,8 @@ void GUI::loadData(const Data &data)
 
 	for (int i = 0; i < _tabs.count(); i++)
 		graphs.append(_tabs.at(i)->loadData(data, _map));
+	/* Refreshing the splitter is necessary to update the map viewport and
+	   properly fit the data! */
 	if (updateGraphTabs())
 		_splitter->refresh();
 	paths = _mapView->loadData(data);

src/main.cpp

@@ -12,6 +12,8 @@ int main(int argc, char *argv[])
 	qRegisterMetaTypeStreamOperators<TimeZoneInfo>("TimeZoneInfo");
 #else // QT6
 	qRegisterMetaType<TimeZoneInfo>("TimeZoneInfo");
+	QGuiApplication::setHighDpiScaleFactorRoundingPolicy(
+	  Qt::HighDpiScaleFactorRoundingPolicy::Round);
 #endif // QT6
 
 	QSurfaceFormat fmt;