migrate to mapbox's expression

[wordgold]

maptalks v0.40.4

chrome v67.0.3396.99

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数据：

{"type":"Feature","geometry":{"coordinates":[87.416234,43.888077],"type":"Point"},"properties":{"t":"头屯河公园","z":15}}

过滤器：

filter: ['>=', ['zoom'],['number', ['get', 'z']]]

期望：根据 z 值通过 filter 设置显示层级

结果：没有起作用，并且破坏了原有的 symbol 设置

过滤器已在mapbox验证过

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期望： 放大后： 结果：

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