采用双缓冲，解决自定控件闪烁问题

[yzhbest]

优化BaseForm的OnPaint函数
protected override void OnPaint(PaintEventArgs e)
{
//Graphics g = e.Graphics;
//GDIHelper.InitializeGraphics(g);
//this.DrawFormBackGround(g);
//this.DrawCaption(g);
//this.DrawFormBorder(g);

        //缓冲
        BufferedGraphicsContext currentContext = BufferedGraphicsManager.Current;
        BufferedGraphics myBuffer = currentContext.Allocate(e.Graphics, e.ClipRectangle);
        Graphics g = myBuffer.Graphics;
        GDIHelper.InitializeGraphics(g);
        this.DrawFormBackGround(g);
        this.DrawCaption(g);
        this.DrawFormBorder(g);

        g.SmoothingMode = SmoothingMode.HighQuality;
        g.PixelOffsetMode = PixelOffsetMode.HighSpeed;
        myBuffer.Render(e.Graphics);
        g.Dispose();
        myBuffer.Dispose();//释放资源
    }