PartDesign: Recompute preview after forced recompute

This fixes some cases where Preview was stale and not recomputed after
changes done via code.

(cherry picked from commit c10f5d74d3)

src/Mod/PartDesign/Gui/TaskFeatureParameters.cpp

@@ -131,9 +131,11 @@ void TaskFeatureParameters::onUpdateView(bool on)
 void TaskFeatureParameters::recomputeFeature()
 {
     if (!blockUpdate) {
-        App::DocumentObject* obj = getObject();
-        assert(obj);
-        obj->recomputeFeature();
+        auto* feature = getObject<PartDesign::Feature>();
+        assert(feature);
+
+        feature->recomputeFeature();
+        feature->recomputePreview();
     }
 }