canvas绘制线条1像素的问题

http://jo2.org/html5-canvas%E7%94%BB%E5%9B%BE3%EF%BC%9A1px%E7%BA%BF%E6%9D%A1%E6%A8%A1%E7%B3%8A%E9%97%AE%E9%A2%98/