clearRect改进的小建议

[jerry-TangHao]

clearRect 放大后出现了一点错位，
我测试的canvas是允许透明的，
clearRect 清空的区域是否应该也是透明的

node-canvas 这个库生成的svg效果挺好的
看看能否为你的改进提供一些帮助

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