Issue #2355 - Apply self-transforms to the frame of top level <svg>

We previously didn't do this because we relied on our anonymous wrapper
to perform transforms. However, that resulted in only the children
transforming, and the frame's cliprect wouldn't be updated,
giving the impression of the z-ordering being wrong.

Also adds reftests from the relevant BZ bug found later.

Resolves #2355

layout/reftests/svg/transform-outer-svg-01-ref.svg

@@ -0,0 +1,9 @@
+<!--
+     Any copyright is dedicated to the Public Domain.
+     http://creativecommons.org/publicdomain/zero/1.0/
+-->
+<svg xmlns="http://www.w3.org/2000/svg" width="250" height="250">
+  <!-- From https://bugzilla.mozilla.org/show_bug.cgi?id=1353697 -->
+  <rect x="75" y="75" width="150" height="150" fill="none" stroke="black" stroke-width="50"/>
+  <rect x="125" y="125" width="50" height="50" fill="blue"/>
+</svg>