Merge pull request w3c#554 from flackr/edgemode-mirror

[filter-effects] Add mirror edgeMode filter

Author: chrishtr

filter-effects-2/Overview.bs

@@ -80,7 +80,7 @@ output as its SourceGraphic.
 Filter functions must operate in the sRGB color space.
 
 If the filter functions list includes a <a href="https://drafts.fxtf.org/filter-effects-1/#blurEquivalent">blur()</a>
-filter, the filter will be applied with edgeMode="duplicate", with the edge defined by the clipped, transformed border
+filter, the filter will be applied with edgeMode="mirror", with the edge defined by the clipped, transformed border
 box of the element. See [[#BackdropRoot]].
 
 Note: The effect of the backdrop-filter will not be visible unless some