dash: Don't unnecessarily animate icon size changes

We already skip animations for item additions/removals while the
overview is hidden or when initially filling the dash (to avoid
an odd zoom effect when showing), apply the same logic to animations
of icon size changes.

js/ui/dash.js

@@ -434,6 +434,12 @@ Dash.prototype = {
             // Set the new size immediately, to keep the icons' sizes
             // in sync with this._iconSize
             icon.setIconSize(this._iconSize);
+
+            // Don't animate the icon size change when the overview
+            // is not visible or when initially filling the dash
+            if (!Main.overview.visible || !this._shownInitially)
+                continue;
+
             let [targetWidth, targetHeight] = icon.icon.get_size();
 
             // Scale the icon's texture to the previous size and