These are some reftests which cover filter primitive subregion when scaled by page

LayoutTests/css3/filters/effect-reference-subregion-hidpi.html

Index: LayoutTests/css3/filters/effect-reference-subregion-hidpi.html
diff --git a/LayoutTests/css3/filters/effect-reference-subregion-hidpi.html b/LayoutTests/css3/filters/effect-reference-subregion-hidpi.html
new file mode 100644
index 0000000000000000000000000000000000000000..e701d29cd6d4cfeecd8f36343f86f1963b5aed1c
--- /dev/null
+++ b/LayoutTests/css3/filters/effect-reference-subregion-hidpi.html
@@ -0,0 +1,27 @@
+<script>
+    if (window.testRunner) {
+        testRunner.waitUntilDone();
+        testRunner.setBackingScaleFactor(2, function() { testRunner.notifyDone(); });
+    }
+</script>
+<svg xmlns="http://www.w3.org/2000/svg" width="0" height="0" version="1.1">
+  <defs>
+    <filter id="colormatrix" color-interpolation-filters="sRGB">
+      <feColorMatrix x="10" y="20" width="80" height="60"/>
+    </filter>
+  </defs>
+</svg>
+<style>
+.filtered {
+    -webkit-filter: url(#colormatrix);
+    filter: url(#colormatrix);
+    background-color: green;
+    width: 100px;
+    height: 100px;
+}
+body {
+    margin: 0px;
+}
+</style>
+<div class="filtered">
+</div>