fitExtent doesn't work at the second time

[fuzhenn]

maptalks's version and what browser you use?

v1.0.0-beta.7

Issue description

fitExtent doesn't work at the second time

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

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`var map = new maptalks.Map('map', { center: [-0.113049, 51.498568], zoom: 14, baseLayer: new maptalks.TileLayer('base', { urlTemplate: 'https://{s}.basemaps.cartocdn.com/light_all/{z}/{x}/{y}.png', subdomains: ['a', 'b', 'c', 'd'], attribution: '© OpenStreetMap contributors, © CARTO ' }), layers: [ new maptalks.VectorLayer('v') ] });

var center = map.getCenter(); var polygon = new maptalks.Polygon([ center.add(-0.005, 0.005), center.add(0.005, 0.005), center.add(0.005, -0.005), center.add(-0.005, -0.005) ], { symbol: { polygonFill: '#fff', polygonOpacity: 0.5 } }); map.getLayer('v').addGeometry(polygon);

function fitExtent() { // fit map's extent to polygon's // 0 is the zoom offset map.fitExtent(polygon.getExtent(), 0); map.setZoom(15); }`

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https://user-images.githubusercontent.com/50706050/147749831-7e163c43-6aef-4496-918f-563dc8942873.mp4

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https://codepen.io/pzd2937/pen/abLqaBj