怎么用Flutter自定义一个图片预览页面，具有下滑关闭，点击关闭，放大缩小等功能。
该项目已废弃，请使用preimage代替。
preimage修复了如果在下拉关闭的时候，如果图片有位移或者缩放的时候，会有一定几率出现下拉漂移的问题。
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