Bug 979489. Add an additional pixel of fuzz to border-image-element test

Without this per-tile drawing fails presumably because of slightly different
floating point someplace.

--HG--
extra : rebase_source : 8c19191a8f7cd1a31d46eccbb3b807537fb3296e

layout/reftests/border-image/reftest.list

@@ -79,4 +79,4 @@ fails-if(OSX) == border-image-repeating-radial-gradient-repeat-repeat-2.html bor
 fuzzy(1,1054) fails-if(OSX) == border-image-repeating-radial-gradient-repeat-round-2.html border-image-repeating-radial-gradient-repeat-round-2-ref.html
 
 # border-image-source (-moz-)element
-fuzzy(125,5807) == border-image-element.html border-image-element-ref.html
+fuzzy(125,5808) == border-image-element.html border-image-element-ref.html