gtk: Fix Builder xml definition loading

Gtk.Builder crashes when AddFromString(string) is called with a string
that contains a BOM. Hence, if the buffer contains a BOM, just skip it.

This fixes the crash with the Gtk# 3 project template in MonoDevelop
(robpvn/MonoDevelopGtkSharp3Template).

gtk/Builder.cs

@@ -26,8 +26,10 @@
 namespace Gtk {
 
 	using System;
+	using System.IO;
 	using System.Reflection;
 	using System.Runtime.InteropServices;
+	using System.Text;
 
 	public partial class Builder {
 
@@ -159,13 +161,7 @@ namespace Gtk {
 			if (s == null)
 				throw new ArgumentNullException ("s");
 		
-			int size = (int) s.Length;
+			AddFromStream (s);
-			byte[] buffer = new byte[size];
-			s.Read (buffer, 0, size);
-			s.Close ();
-		
-			AddFromString(System.Text.Encoding.UTF8.GetString (buffer));
-		
 			TranslationDomain = translation_domain;
 		}
 		
@@ -191,13 +187,7 @@ namespace Gtk {
 				throw new ArgumentException ("Cannot get resource file '" + resource_name + "'",
 				                             "resource_name");
 		
-			int size = (int) s.Length;
+			AddFromStream (s);
-			byte[] buffer = new byte[size];
-			s.Read (buffer, 0, size);
-			s.Close ();
-		
-			AddFromString(System.Text.Encoding.UTF8.GetString (buffer));
-		
 			TranslationDomain = translation_domain;
 		}
 		
@@ -368,6 +358,22 @@ namespace Gtk {
 		
 		}
 		
+		void AddFromStream (Stream stream)
+		{
+			var size = (int)stream.Length;
+			var buffer = new byte[size];
+			stream.Read (buffer, 0, size);
+			stream.Close ();
+
+			// If buffer contains a BOM, omit it while reading, otherwise AddFromString(text) crashes
+			var offset = 0;
+			if (size >= 3 && buffer [0] == 0xEF && buffer [1] == 0xBB && buffer [2] == 0xBF) {
+				offset = 3;
+			}
+
+			var text = Encoding.UTF8.GetString (buffer, offset, size - offset);
+			AddFromString (text);
+		}
 		
 		void BindFields (object target)
 		{