Add percent support for svgSize

test struct-frag-02-t-manual.svg
```
<svg version="1.1" baseProfile="tiny" preserveAspectRatio="none" id="svg-root"
  width="100%" height="100%" viewBox="100 100 100 100"
  xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
```