支持画笔及痕迹擦除，为白板做准备

AIStudio.Wpf.DiagramDesigner/Converters/NegativeConverter.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+using System.Windows.Data;
+
+namespace AIStudio.Wpf.DiagramDesigner
+{
+    public class NegativeConverter : IValueConverter
+    {
+        public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
+        {
+            if (value is double)
+            {
+                return 0 - (double)value;
+            }
+
+            return value;
+        }
+
+        public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
+        {
+            throw new NotImplementedException();
+        }
+    }
+}