QScreen::refreshRate would return 0 on Mac OS X

The function we use to get the actual vsync on cocoa is documented
to return 0 if the monitor is not a CRT monitor. A refreshrate
of 0 means we have vsync deltas of 1000/0 which cause problems
elsewhere. It is better to use the "default" value in this case
as it will be closer to correct than 0.

Change-Id: Id08007e40a9af5e42f13a07628fcad5fd3a7d0dc
Reviewed-by: Morten Johan Sørvig <morten.sorvig@digia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>

src/plugins/platforms/cocoa/qcocoaintegration.mm

@@ -105,7 +105,9 @@ void QCocoaScreen::updateGeometry()
     m_physicalSize = QSizeF(size.width, size.height);
     m_logicalDpi.first = 72;
     m_logicalDpi.second = 72;
-    m_refreshRate = CGDisplayModeGetRefreshRate(CGDisplayCopyDisplayMode(dpy));
+    float refresh = CGDisplayModeGetRefreshRate(CGDisplayCopyDisplayMode(dpy));
+    if (refresh > 0)
+        m_refreshRate = refresh;
 
     // Get m_name (brand/model of the monitor)
     NSDictionary *deviceInfo = (NSDictionary *)IODisplayCreateInfoDictionary(CGDisplayIOServicePort(dpy), kIODisplayOnlyPreferredName);