ImageLayer的半透明被设置了两次

**maptalks's version and what browser you use?**
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maptalks@1.0.0-rc.37

**Issue description**
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ImageLayer设置opacity时，会出现叠加，比如设置 opacity: 0.5，实际生效的是 0.5*0.5=0.25

**Please provide a reproduction URL (on any jsfiddle like site)**
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https://codepen.io/ydfzgyj/pen/QWPywOW