Fix painting of fixed background images in composited layers

LayoutTests/compositing/backgrounds/fixed-background-on-descendant-expected.html

+<style>

[esprehn, 2014/04/01 22:00:13]

same

+    body {
+        height: 2000px;
+        width: 2000px;
+        overflow: hidden;
+    }
+
+    .box {
+        width: 300px;
+        height: 300px;
+        margin: 20px;
+        position: relative;
+    }
+
+    body > .box {
+        border: 1px solid black;
+        float: left;
+    }
+    .child {
+        position: absolute;
+        top: 50px;
+        left: 50px;
+        height: 200px;
+        width: 200px;
+        margin-left: 10px;
+        background-image: url('../resources/alpha-blocks.png');
+        background-size: 500px 300px;
+        background-repeat: no-repeat;
+        background-attachment: fixed;
+        border: 4px solid gray;
+    }
+</style>
+<script>
+    function doTest()
+    {
+        window.scrollTo(35, 45);
+    }
+    window.addEventListener('load', doTest, false);
+</script>
+<div class="box">
+    <div class="child box"></div>
+</div>
+
+<div class="box">
+    <div class="child box"></div>
+</div>