Nested styling has lower priority than referential styling #191
Description
Referential styling has (incorrectly) higher priority than nested styling.
In the following the background color of region r2 should be yellow:
<?xml version="1.0" encoding="UTF-8"?>
<tt xml:lang="en"
xmlns="http://www.w3.org/ns/ttml"
xmlns:tts="http://www.w3.org/ns/ttml#styling"
xmlns:ttp="http://www.w3.org/ns/ttml#parameter"
ttp:contentProfiles="http://www.w3.org/ns/ttml/profile/imsc1.1/text">
<head>
<styling>
<style xml:id="s1" tts:color="red" tts:backgroundColor="blue"/>
<style xml:id="s2" tts:color="yellow" style="s1"/>
<style xml:id="s3" style="s2 s1"/>
<style xml:id="s4" tts:color="green" style="s3"/>
</styling>
<layout>
<region xml:id="r1" style="s3 s1" tts:extent="100% 50%" tts:color="blue" tts:backgroundColor="yellow"/>
<region xml:id="r2" style="s1" tts:origin="0% 50%" tts:extent="100% 50%" tts:color="red">
<style tts:color="blue" tts:backgroundColor="yellow"/>
</region>
</layout>
</head>
<body>
<div region="r1"><p>one</p></div>
<div region="r2"><p>two</p></div>
</body>
</tt>