[FIX] PluginImg: $link_button needs filter_out_sefurl too sometimes, as was done for $link and $src in r22212 and r43231

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/12.x@49582 b456876b-0849-0410-b77d-98878d47e9d5