桌面环境：增加环境变量APP并给Paint应用程序增加更多画图工具

Gui/Apps/Paint/Tools/Eraser.cs

@@ -0,0 +1,53 @@
+using Cosmos.System;
+using System.Drawing;
+
+namespace CMLeonOS.Gui.Apps.Paint.Tools
+{
+    internal class Eraser : Tool
+    {
+        public Eraser() : base("Eraser")
+        {
+        }
+
+        private bool joinLine;
+        private int joinX;
+        private int joinY;
+
+        internal override void Run(Paint paint, Window canvas, MouseState mouseState, int mouseX, int mouseY)
+        {
+            if (mouseState == MouseState.Left)
+            {
+                if (joinLine)
+                {
+                    canvas.DrawLine(joinX, joinY, mouseX, mouseY, Color.White);
+                }
+
+                for (int y = -4; y <= 4; y++)
+                {
+                    for (int x = -4; x <= 4; x++)
+                    {
+                        int drawX = mouseX + x;
+                        int drawY = mouseY + y;
+                        if (paint.IsInBounds(drawX, drawY))
+                        {
+                            canvas.DrawPoint(drawX, drawY, Color.White);
+                        }
+                    }
+                }
+
+                joinLine = true;
+                joinX = mouseX;
+                joinY = mouseY;
+            }
+            else
+            {
+                joinLine = false;
+            }
+        }
+
+        internal override void Deselected()
+        {
+            joinLine = false;
+        }
+    }
+}