cssimage: Scale images to contain, not cover

Images with just an aspect ratio, but without a size, should be scaled
to be fully visible in the given area.
But we scaled them to completely cover the given area, which made them
partially invisible.

Reftest included.

testsuite/reftests/css-image-aspect-ratio.ref.ui

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<interface>
+  <!-- interface-requires gtk+ 3.0 -->
+  <object class="GtkWindow" id="window1">
+    <property name="width_request">48</property>
+    <property name="height_request">24</property>
+    <property name="can_focus">False</property>
+    <property name="type">popup</property>
+    <property name="name">reference</property>
+  </object>
+</interface>