Fix infinite loop, specificity bug

[Randalphwa]

This PR fixes a infinite loop when processing some SVG files. Fixing that loop also exposed an additional bug that caused the test svg fail to render incorrectly. The fix adds pseudo-class selectors to the specificity calculation with weight 0x100 (same as class and attribute selectors per CSS spec). Now rect:first-of-type has specificity 0x101 (element + pseudo-class) while plain rect has 0x1, so the first rectangle will correctly be red.

Test SVG

<?xml version="1.0" encoding="UTF-8"?>
<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 200 100">
  <style>
    rect:first-of-type { fill: red; }
    rect { fill: blue; }
  </style>
  <rect x="10" y="10" width="80" height="80"/>
  <rect x="110" y="10" width="80" height="80"/>
</svg>

[sammycage]

Thank you <3