The fill() method closes any subpaths and then fills each subpath in turn using fillStyle (which can be a color, or, as discussed later, a gradient object) to determine how to draw the fill.
fill() 方法使所有子路径封闭起来，然后逐个填充每个子路径，期间使用 fillStyle （一种颜色，或一个渐变对象）来决定如何绘制填充。
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