Support Nested SVG

[swhitty]

Adds support for nested <svg> including support for <use lookups from the parent space.

This sample SVG has been added for testing

<?xml version="1.0" encoding="utf-8"?>
<svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="360" height="450" stroke="black" fill-opacity="0.5">
   <defs>
      <polygon id="star" points="100,10 40,198 190,78, 10,78, 160,198" fill-rule="nonzero" />
   </defs>
	 
   <rect width="360" height="450" fill="snow" />

   <g transform="translate(170, 30) scale(800)" fill="orange">
      <polygon points="-0.1,0.01 -0.04,0.198 -0.190,0.078, -0.010,0.078, -0.160,0.198" stroke="black" stroke-width="0.002" fill-rule="nonzero" />
   </g>

   <svg x="180" y="28" width="165" height="165" viewBox="-0.2 0 0.2 0.2" >
      <polygon fill="green" points="-0.1,0.01 -0.04,0.198 -0.190,0.078, -0.010,0.078, -0.160,0.198" stroke-width="0.002" fill-rule="nonzero" />
   </svg>
	
   <g transform="rotate(28)" stroke="blue">
      <svg x="130" y="130" width="165" height="165" viewBox="-0.2 0 0.2 0.2">
         <polygon fill="red" fill-opacity="0.4" points="-0.1,0.01 -0.04,0.198 -0.190,0.078, -0.010,0.078, -0.160,0.198" stroke="currentColor" stroke-width="0.002" fill-rule="nonzero" />
      </svg>
   </g>
	
   <svg x="0" y="210" width="200" height="200">
      <use xlink:href="#star" fill="blue" stroke-width="2" transform="scale(0.85)" />
   </svg>
	
   <g transform="scale(0.85)">
      <svg x="200" y="240" width="200" height="200">
         <defs>
            <circle id="star" cx="100" cy="110" r="50" fill="purple" stroke-width="2" />
            <circle id="round" cx="100" cy="110" r="50" fill="purple" stroke-width="2" />
         </defs>
         <use xlink:href="#round" />
         <use xlink:href="#star" />
         <circle cx="100" cy="120" r="160" fill="none" stroke-width="2" />
      </svg>
   </g>
</svg>

With SwiftDraw outputting this PNG@2x

As reported in #66

[codecov]

Codecov Report

Attention: Patch coverage is 65.21739% with 16 lines in your changes missing coverage. Please review.

Project coverage is 84.87%. Comparing base (3d021b8) to head (15e34dc).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
SwiftDraw/Parser.XML.Element.swift	10.00%	9 Missing ⚠️
SwiftDraw/LayerTree.Builder.swift	77.41%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #67      +/-   ##
==========================================
- Coverage   85.02%   84.87%   -0.16%     
==========================================
  Files          78       78              
  Lines        7191     7225      +34     
==========================================
+ Hits         6114     6132      +18     
- Misses       1077     1093      +16     

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