reftests: Add a test for misrendering of transparent windows

If a widget with a GDK window is set to be transparent in the CSS, it
gets drawn as black instead. This test checks that the fix works.

tests/reftests/Makefile.am

@@ -43,6 +43,9 @@ EXTRA_DIST += \
 	background-size-keywords.css \
 	background-size-keywords.ref.ui \
 	background-size-keywords.ui \
+	background-window-transparent.css \
+	background-window-transparent.ref.ui \
+	background-window-transparent.ui \
 	reference-background-origin.png	\
 	border-color-default.css \
 	border-color-default.ref.ui \

tests/reftests/background-window-transparent.css

@@ -0,0 +1,5 @@
+@import "reset-to-defaults.css";
+
+GtkWindow { background-color: green; }
+
+#reference * { background-color: green; }

tests/reftests/background-window-transparent.ref.ui

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<interface>
+  <!-- interface-requires gtk+ 3.0 -->
+  <object class="GtkWindow" id="window1">
+    <property name="can_focus">False</property>
+    <property name="type">popup</property>
+    <property name="name">reference</property>
+    <child>
+      <object class="GtkViewport" id="viewport1">
+        <property name="visible">True</property>
+        <property name="can_focus">True</property>
+      </object>
+    </child>
+  </object>
+</interface>

tests/reftests/background-window-transparent.ui

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<interface>
+  <!-- interface-requires gtk+ 3.0 -->
+  <object class="GtkWindow" id="window1">
+    <property name="can_focus">False</property>
+    <property name="type">popup</property>
+    <child>
+      <object class="GtkViewport" id="viewport1">
+        <property name="visible">True</property>
+        <property name="can_focus">True</property>
+      </object>
+    </child>
+  </object>
+</interface>