Fixed ArgumentException when selecting right-to-left text in AvalonEdit.

git-svn-id: svn://svn.sharpdevelop.net/sharpdevelop/trunk@5420 1ccf3a8d-04fe-1044-b7c0-cef0b8235c61

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Rendering/BackgroundGeometryBuilder.cs

@@ -130,7 +130,8 @@ namespace ICSharpCode.AvalonEdit.Rendering
 					y -= scrollOffset.Y;
 					left -= scrollOffset.X;
 					right -= scrollOffset.X;
-					yield return new Rect(left, y, right - left, line.Height);
+					// left>right is possible in RTL languages
+					yield return new Rect(Math.Min(left, right), y, Math.Abs(right - left), line.Height);
 				}
 			}
 		}