New arcs no longer slices before mouse let go

This should reduce reliance on the default constructor.
Introduced by 316d4c9b

(cherry-picked from c7ef69f4)

src/object/sp-ellipse.cpp

@@ -682,6 +682,9 @@ void SPGenericEllipse::position_set(gdouble x, gdouble y, gdouble rx, gdouble ry
     }
 
     this->arc_type = (GenericEllipseArcType)prefs->getInt("/tools/shapes/arc/arc_type", 0);
+    if (_isSlice()) {
+        this->type = SP_GENERIC_ELLIPSE_ARC;
+    }
 
     this->requestDisplayUpdate(SP_OBJECT_MODIFIED_FLAG);
 }