SVGLoader: recognize style definitions inside defs tag

[nkrkv]

Hello! The current implementation of SVGLoader never traverses nodes inside <defs> tag. But besides geometry, it may contain CSS style definitions that should be accessible to paths outside the <defs>.

Here's an SVG (included in the PR):

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<svg
   width="300"
   height="300"
   viewBox="0 0 79.374998 79.375002"
   version="1.1"
   id="svg5"
   xmlns="http://www.w3.org/2000/svg"
   xmlns:svg="http://www.w3.org/2000/svg">
  <defs id="defs2">
     <style type="text/css">
      <![CDATA[
       .str0 {stroke:#FF0000;stroke-width:3}
       .fil0 {fill:#FF9900}
      ]]>
     </style>
  </defs>
  <g
     id="layer1">
    <path
       class="str0 fil0"
       id="path848"
       d="m 137.38278,139.05634 -2.47931,71.54147 62.00995,43.49145 -68.80613,19.74955 L 105.9066,346.25338 65.861386,286.91781 -9.8693534,288.18103 34.18748,231.76007 9.5839998,160.12621 76.857831,184.59172 Z"
       transform="matrix(0.26458333,0,0,0.26458333,15.953737,-22.418478)" />
  </g>
</svg>

This is a simplified example of typical Adobe Illustrator and CorelDRAW output: they put styles in <defs>. This SVG is correctly rendered in yellow + red with Firefox and Chrome, but incorrectly with three.js:

This patch does the fix:

[yomboprime]

Seems right to me. Thanks!

[yomboprime]

@mrdoob this seems right to be merged.

[mrdoob]

Thanks!