ScreenshotEffect: fix lookup of ComparableQPoint keys in QMap

QMap::value() failed to find existing keys when tow ComparablePoint
shared the same x but not the same y.

This commit improves the operator<() to fix that.

effects/screenshot/screenshot.cpp

@@ -48,7 +48,7 @@ public:
     // utility class that allows using QMap to sort its keys when they are QPoint
     // so that the bottom and right points are after the top left ones
     bool operator<(const ComparableQPoint &other) const {
-        return x() < other.x() || y() < other.y();
+        return x() < other.x() || (x() == other.x() && y() < other.y());
     }
 };