QEasingCurve: fix missing copy() override

TCBEase is-a BezierEase, but overrides value(). It only performs additional
checks and then calls the base class' implementation, but it feels wrong,
like a bug waiting to manifest itself, that slicing should occur on cloning
a TCBEase.

Fix by adding the missing reimplmentation.

Change-Id: I2524f51fec1850ff36ed706bc79e9592734d8680
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>

src/corelib/tools/qeasingcurve.cpp

@@ -907,6 +907,10 @@ struct TCBEase : public BezierEase
         return BezierEase::value(x);
     }
 
+    QEasingCurveFunction *copy() const override
+    {
+        return new TCBEase{*this};
+    }
 };
 
 struct ElasticEase : public QEasingCurveFunction