屏幕抓屏（包括活动窗口，指定区域及全屏等）

using  System;
using  System.Runtime.InteropServices;
using  System.Drawing;

namespace  SystemTools
... {
    [StructLayout(LayoutKind.Sequential)]
    public struct RECT
    ...{
        public int left, top, right, bottom;
    }

    /**//// <summary>
    /// 封装API的类
    /// </summary>
    public class APIs
    ...{
        [DllImport("user32.dll")]
        public static extern bool GetCursorPos(ref Point lpPoint);
        [DllImport("user32.dll")]
        public static extern int WindowFormPoint(Point lpPoint);
        [DllImport("user32.dll")]
        public static extern IntPtr GetDesktopWindow();
        [DllImport("user32.dll")]
        public static extern int GetForegroundWindow();
        [DllImport("user32.dll")]
        public static extern int GetWindowRect(int hwnd, ref RECT rc);
        [DllImport("user32.dll")]
        public static extern int GetWindowDC(int hwnd);
        [DllImport("Gdi32.dll")]
        public