Lock GDI+ portion og VectorRenderModule.GetDrawStringSize() to prevent concurrent thread use provoking mono crashes.

Same rationale as commit 13690582.

OpenSim/Region/CoreModules/Scripting/VectorRender/VectorRenderModule.cs

@@ -97,10 +97,14 @@ namespace OpenSim.Region.CoreModules.Scripting.VectorRender
 
         public void GetDrawStringSize(string text, string fontName, int fontSize, 
                                       out double xSize, out double ySize)
+        {
+            lock (this)
             {
                 using (Font myFont = new Font(fontName, fontSize))
                 {
                     SizeF stringSize = new SizeF();
+
+                    // XXX: This lock may be unnecessary.
                     lock (m_graph)
                     {
                         stringSize = m_graph.MeasureString(text, myFont);
@@ -109,6 +113,7 @@ namespace OpenSim.Region.CoreModules.Scripting.VectorRender
                     }
                 }
             }
+        }
 
         #endregion