Fix issue with percentage unit in "stroke-width", now taken from the viewbox...

Fix issue with percentage unit in "stroke-width", now taken from the viewbox width/height normalised diagonale for x/y elements e.g. circle
Test images and svg under #338

Source/DataTypes/SvgUnit.cs

@@ -164,6 +164,14 @@ namespace Svg
                         case UnitRenderingType.VerticalOffset:
                             _deviceValue = (size.Height / 100) * value + boundable.Location.Y;
                             break;
+                        case UnitRenderingType.Other:
+                            // Calculate a percentage value of the normalized viewBox diagonal length. 
+                            if (owner.OwnerDocument != null && owner.OwnerDocument.ViewBox != null && owner.OwnerDocument.ViewBox.Width!=0 && owner.OwnerDocument.ViewBox.Height != 0)
+                            {
+                                _deviceValue = (float)(Math.Sqrt(Math.Pow(owner.OwnerDocument.ViewBox.Width, 2) + Math.Pow(owner.OwnerDocument.ViewBox.Height, 2)) / Math.Sqrt(2) * value / 100.0);
+                            }
+                            else _deviceValue = (float)(Math.Sqrt(Math.Pow(size.Width, 2) + Math.Pow(size.Height, 2)) / Math.Sqrt(2) * value / 100.0);
+                            break;
                         default:
                             _deviceValue = (float)(Math.Sqrt(Math.Pow(size.Width, 2) + Math.Pow(size.Height, 2)) / Math.Sqrt(2) * value / 100.0);
                             break;

Tests/W3CTestSuite/svg/__issue-338-01_stroke_width.svg

+<svg width="600" height="200" viewBox="0 0 30 10" xmlns="http://www.w3.org/2000/svg">
+  <!-- Default stroke width: 1 -->
+  <circle cx="5" cy="5" r="3" stroke="green"
+          stroke-width="1"  />
+
+  <!-- Stroke width as a number -->
+  <circle cx="15" cy="5" r="3" stroke="green"
+          stroke-width="0.5" />
+
+  <!-- Stroke width as a percentage -->
+  <circle cx="25" cy="5" r="3" stroke="green"
+          stroke-width="2%" />
+</svg>
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