fix: 修复 2.x 与 1.x 下相同的网格线宽，Grid 表现不一致的问题

[ChangeSuger]

#1904

重写 Grid 的时候没有考虑到原先只有一半的宽度可见，实际表现就是 2.x 的网格在相同的网格线宽下，线的宽度实际上是 1.x 的两倍。举个例子，使用下面的配置，在 1.x 下，网格的线宽实际上是 stroke-width="0.5"；而在 2.x 下，网格的线宽实际上是 stroke-width="1"。

const gridConfig = {
  type: 'mesh',
  config: {
    thickness: 1
  }
}

考虑到迁移影响，这里就将 2.x 的表现调整为与 1.x 的表现一致。

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