SVG 阴影滤镜：基本的阴影
示例
<svg width="800px" height="600px">
<defs>
<filter id="drop-shadow">
<feGaussianBlur in="SourceAlpha" stdDeviation="4"/>
<feOffset dx="5" dy="5" result="offsetblur"/>
<feFlood flood-color="red"/>
<feComposite in2="offsetblur" operator="in"/>
<feMerge>
<feMergeNode/>
<feMergeNode in="SourceGraphic"/>
</feMerge>
</filter>
</defs>
<text filter="url(#drop-shadow)" x="30" y="100" font-size="80">SVG Filters</text>
</svg>