Added support for marker attributes in groups

- partly fixes respective W3C examples

Source/Basic Shapes/SvgMarkerElement.cs

@@ -20,7 +20,7 @@ namespace Svg
 
 
         /// <summary>
-        /// Gets or sets the marker (start cap) of the path.
+        /// Gets or sets the marker (mid points) of the path.
         /// </summary>
         [SvgAttribute("marker-mid", true)]
         public Uri MarkerMid
@@ -41,7 +41,8 @@ namespace Svg
         }
 
         /// <summary>
-        /// Renders the stroke of the <see cref="SvgVisualElement"/> to the specified <see cref="ISvgRenderer"/>
+        /// Renders the stroke of the element to the specified <see cref="ISvgRenderer"/>.
+        /// Includes rendering of all markers defined in attributes.
         /// </summary>
         /// <param name="renderer">The <see cref="ISvgRenderer"/> object to render to.</param>
         protected internal override bool RenderStroke(ISvgRenderer renderer)

Source/Document Structure/SvgGroup.cs

+using Svg.ExtensionMethods;
+using System;
 using System.Drawing;
-using System.Drawing.Drawing2D;
 
 namespace Svg
 {
@@ -7,8 +8,54 @@ namespace Svg
     /// An element used to group SVG shapes.
     /// </summary>
     [SvgElement("g")]
-    public class SvgGroup : SvgVisualElement
+    public class SvgGroup : SvgMarkerElement
     {
+        bool markersSet = false;
+
+        /// <summary>
+        /// If the group has marker attributes defined, add them to all children
+        /// that are able to display markers. Only done once.
+        /// </summary>
+        private void AddMarkers()
+        {
+            if (!markersSet)
+            {
+                if (this.MarkerStart != null || this.MarkerMid != null || this.MarkerEnd != null)
+                {
+                    foreach (var c in this.Children)
+                    {
+                        if (c is SvgMarkerElement)
+                        {
+                            if (this.MarkerStart != null && ((SvgMarkerElement)c).MarkerStart == null)
+                            {
+                                ((SvgMarkerElement)c).MarkerStart = this.MarkerStart;
+                            }
+                            if (this.MarkerMid != null && ((SvgMarkerElement)c).MarkerMid == null)
+                            {
+                                ((SvgMarkerElement)c).MarkerMid = this.MarkerMid;
+                            }
+                            if (this.MarkerEnd != null && ((SvgMarkerElement)c).MarkerEnd == null)
+                            {
+                                ((SvgMarkerElement)c).MarkerEnd = this.MarkerEnd;
+                            }
+                        }
+                    }
+                }
+                markersSet = true;
+            }
+        }
+
+        /// <summary>
+        /// Add group markers to children before rendering them.
+        /// This is only done on first rendering.
+        /// </summary>
+        /// <param name="renderer">The <see cref="ISvgRenderer"/> to render the child <see cref="SvgElement"/>s to.</param>
+        protected override void Render(ISvgRenderer renderer)
+        {
+            AddMarkers();
+            base.Render(renderer);
+        }
+
         /// <summary>
         /// Gets the <see cref="GraphicsPath"/> for this element.
         /// </summary>