Fix warnings in gradients.

We're assigning 'undefined' to it, so implement a resetter.

Change-Id: Ie43282043b250f2e9cd247d4819b1827ab7e9e97
Task-number: QTBUG-46774
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>

src/effects/private/qgfxsourceproxy_p.h

@@ -44,7 +44,7 @@ class QGfxSourceProxy : public QQuickItem
 {
     Q_OBJECT
 
-    Q_PROPERTY(QQuickItem *input READ input WRITE setInput NOTIFY inputChanged)
+    Q_PROPERTY(QQuickItem *input READ input WRITE setInput NOTIFY inputChanged RESET resetInput)
     Q_PROPERTY(QQuickItem *output READ output NOTIFY outputChanged)
     Q_PROPERTY(QRectF sourceRect READ sourceRect WRITE setSourceRect NOTIFY sourceRectChanged)
 
@@ -65,6 +65,7 @@ public:
 
     QQuickItem *input() const { return m_input; }
     void setInput(QQuickItem *input);
+    void resetInput() { setInput(0); }
 
     QQuickItem *output() const { return m_output; }