iconhelper: Improve size determination for surfaces

Instead of just working for image surface, this should now work for all
bounded surfaces.

Test included.

testsuite/reftests/image-recording-surface.ref.ui

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- Generated with glade 3.19.0 -->
+<interface>
+  <requires lib="gtk+" version="3.16"/>
+  <object class="GtkWindow" id="window1">
+    <property name="can_focus">False</property>
+    <property name="type">popup</property>
+    <child>
+      <object class="GtkImage" id="image1">
+        <property name="visible">True</property>
+        <property name="can_focus">False</property>
+        <property name="resource">/org/gtk/libgtk/inspector/logo.png</property>
+      </object>
+    </child>
+  </object>
+</interface>