Result attributes removed from filter

[zwaldowski]

I use a drop shadow filter:

<filter id="drop-shadow">
    <feGaussianBlur stdDeviation="8" result="a"/>
    <feFlood flood-color="#000" flood-opacity=".75" result="b"/>
    <feComposite operator="in" in2="a" in="b"/>
</filter>

SVGO mangles it:

<filter id="a">
    <feGaussianBlur stdDeviation="8"/>
    <feFlood flood-color="#000" flood-opacity=".75"/>
    <feComposite operator="in" in2="a" in="b"/>
</filter>

If there's any way I can make SVGO's version work, I'd love to hear it. Awesome work, going around and shaving large chunks off my web SVGs.