Fix bug 603994, whereby a <style> stylesheet would not count as a reference towards gradients etc., if the stylesheet was a CDATA element surrounded by whitespace nodes.

Add unit tests for this.

Add some descriptions to the failure modes of some unit tests, which would fail without any message (if they failed, but right now they succeed! ;).

unittests/style-cdata.svg

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="200" height="200">
+	<defs>
+		<style type="text/css">
+			<![CDATA[
+				rect { fill: url(#somethingreallylong); }
+			]]>
+		</style>
+		<linearGradient id="somethingreallylong" x1="0" x2="0" y1="0" y2="1">
+			<stop stop-color="red" offset="0" />
+			<stop stop-color="green" offset="1" />
+		</linearGradient>
+	</defs>
+
+	<rect width="100" height="100" />
+</svg>