Fix for disallowed property bindings in qml effect files

According to the documentation of ListModel.get, "The returned object
is not guaranteed to remain valid. It should not be used in property
bindings." So the property bindings have been removed, and a js function
has been introduced to update the properties upon list change.

Task-number: QTBUG-49221
Change-Id: Idaed746ca237198b52a3aff0234076331e1512c1
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>

examples/multimedia/video/qmlvideofx/qml/qmlvideofx/EffectPosterize.qml

@@ -39,10 +39,16 @@ Effect {
             name: "Gamma"
             value: 0.5
         }
+        onDataChanged: updateParameters()
+    }
+
+    function updateParameters()
+    {
+            gamma = parameters.get(0).value;
     }
 
     // Transform slider values, and bind result to shader uniforms
-    property real gamma: parameters.get(0).value
+    property real gamma: 0.5
 
     property real numColors: 8.0