Do not create a clip region for view box

- a view box does not need a clip region, this breaks the overflow
handling
- overflow handling:  handle overflow attribut "Inherit" as Visible and
"Scroll" as Hidden as per W3C documentation
- fixes #279

Source/DataTypes/SvgViewBox.cs

@@ -200,7 +200,6 @@ namespace Svg
                 }
             }
             
-            renderer.SetClip(new Region(new RectangleF(x, y, width, height)), CombineMode.Intersect);
             renderer.TranslateTransform(x, y, MatrixOrder.Prepend);
             renderer.TranslateTransform(fMinX, fMinY, MatrixOrder.Prepend);
             renderer.ScaleTransform(fScaleX, fScaleY, MatrixOrder.Prepend);       

Source/Document Structure/SvgFragment.cs

@@ -165,7 +165,7 @@ namespace Svg
             {
                 case SvgOverflow.Auto:
                 case SvgOverflow.Visible:
-                case SvgOverflow.Scroll:
+                case SvgOverflow.Inherit:
                     base.Render(renderer);
                     break;
                 default:

Tests/W3CTestSuite/svg/__issue-279-01.svg

+<svg version="1.1" baseProfile="basic" width="480" height="360"
+  xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+  <title id="test-title">Test data for issue 279 - overflow not shown with viewbox on svg fragment</title>
+  <g fill="lightblue" stroke="black">
+    <svg x="35" y="50" width="100" height="100" viewBox="0 0 150 150" overflow="visible">
+      <path fill-rule="evenodd" d="M20,20 Q180,20 180,180 Q20,180 20,20 Z M20,180 Q20,20 180,20 Q180,180 20,180 Z M100,40 L160,100 100,160 40,100 Z"/>
+    </svg>
+    <text font-size="13.5" x="100" y="190" text-anchor="middle" fill="black" stroke="none">viewBox="0 0 150 150"</text>
+    <text font-size="13.5" x="100" y="205" text-anchor="middle" fill="black" stroke="none">overflow="visible" </text>
+    <svg x="250" y="50" width="100" height="100" viewBox="0 0 150 150" overflow="hidden">
+      <path fill-rule="evenodd" d="M20,20 Q180,20 180,180 Q20,180 20,20 Z M20,180 Q20,20 180,20 Q180,180 20,180 Z M100,40 L160,100 100,160 40,100 Z"/>
+    </svg>
+    <text font-size="13.5" x="315" y="190" text-anchor="middle" fill="black" stroke="none">viewBox="0 0 150 150"</text>
+    <text font-size="13.5" x="315" y="205" text-anchor="middle" fill="black" stroke="none">overflow="hidden" </text>
+  </g>
+</svg>