Only update the IME composition window when the VisualLine for the caret position is available.

This fixes a performance issue while scrolling (without this change, UpdateCompositionWindow() would cause the line containing the caret to be re-generated whenever the user scrolls).

src/Libraries/AvalonEdit/ICSharpCode.AvalonEdit/Editing/Caret.cs

@@ -431,10 +431,10 @@ namespace ICSharpCode.AvalonEdit.Editing
 						Win32.SetCaretPosition(textView, caretRect.Location - textView.ScrollOffset);
 					}
 					caretAdorner.Show(caretRect);
+					textArea.ime.UpdateCompositionWindow();
 				} else {
 					caretAdorner.Hide();
 				}
-				textArea.ime.UpdateCompositionWindow();
 			}
 		}