gtk-demo: Make scrolling benchmark resizable

The small window does not really tax our renderer much. Make it
resizable, so we can test fullscreen scrolling.

demos/gtk-demo/iconscroll.ui

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <interface>
   <object class="GtkWindow" id="window">
-    <property name="resizable">0</property>
+    <property name="resizable">1</property>
     <property name="default-width">500</property>
     <property name="default-height">500</property>
     <child type="titlebar">