Fix ArgumentOutOfRangeException in RichText.Concat() #242

https://github.com/icsharpcode/AvalonEdit/pull/242 (tests to be ported seperately.)

src/AvaloniaEdit/Highlighting/RichTextModel.cs

@@ -157,8 +157,8 @@ namespace AvaloniaEdit.Highlighting
         {
             Debug.Assert(newOffsets.Length == newColors.Length);
             Debug.Assert(newOffsets[0] == 0);
-            // remove everything before offset:
-            while (_stateChangeOffsets.Count > 0 && _stateChangeOffsets.Last() <= offset)
+            // remove everything not before offset:
+            while (_stateChangeOffsets.Count > 0 && _stateChangeOffsets.Last() >= offset)
             {
                 _stateChangeOffsets.RemoveAt(_stateChangeOffsets.Count - 1);
                 _stateChanges.RemoveAt(_stateChanges.Count - 1);