有高度的线设置vertex-firstlast时双向箭头不显示

[cXiaof]

maptalks's version and what browser you use?

maptalks@>=0.49.5 edge

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Issue description

LineString在设置arrow的同时设置了高度的话，箭头为vertex-firstlast时不显示 没有高度时显示正常，设置为first或last时单向箭头也正常显示

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Please provide a reproduction URL (on any jsfiddle like site)

重现codepen

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