canvas 涂鸦画板，支持笔写、手写、鼠标绘图
kuangyx.cn/docs/%E6%96%87%E7%AB%A0/JS%20Lib/canvas%20%E6%B6%82%E9%B8%A6%E7%94%BB%E6%9D%BF.html
tomiaa12/canvasGraffiti