Fixed bad assert (floating point imprecision)

http://community.sharpdevelop.net/forums/t/15797.aspx

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

@@ -899,7 +899,8 @@ namespace ICSharpCode.AvalonEdit.Rendering
 			
 			// number of pixels clipped from the first visual line(s)
 			clippedPixelsOnTop = scrollOffset.Y - heightTree.GetVisualPosition(firstLineInView);
-			Debug.Assert(clippedPixelsOnTop >= 0);
+			// clippedPixelsOnTop should be >= 0, except for floating point inaccurracy.
+			Debug.Assert(clippedPixelsOnTop >= -ExtensionMethods.Epsilon);
 			
 			newVisualLines = new List<VisualLine>();