Add support for TransformRef attribute in SVG module

SVG 1.2 introduced the TransformRef attribute and it would be nice if it was supported in Qt.

"provide a method by which graphical objects can remain fixed in the
viewport without being scaled or rotated. Use cases include thin lines
that do not become fatter on zooming in..."

This section is listed under 7.7 Constrained Transformations,
http://www.w3.org/TR/SVGMobile12/coords.html, and under 7.7.5 The
TransformRef value: "By using the ref() attribute on the transform
attribute it is possible to provide constrained transformations."

Thus, the following svg file syntax should fix the rectangle in the face
of scaling. Please see the Example: ref() transform at the following
w3.org link above.

<svg xml:id="root" viewBox="0 0 100 100" version="1.2"
baseProfile="tiny">
<line x1="0" x2="100" y1="0" y2="100"/>
<rect xml:id="r" transform="ref(svg)"
x="45" y="45" width="10" height="10"/>
</svg>

According to the example, the viewport is (200,200). So the line will
be drawn from (0,0) to (200,200) and the rectangle will be drawn in the
middle at (90,90,20,20). But if you zoom any bigger, keeping the
viewport dimensions at (200,200), the line will grow but the rectangle
should stay at (90,90,20,20).