[bounty] improved whiteout

[Kieren MacMillan]

Hello all,

Here’s my semi-annual request-slash-U$25-bounty-offer for an improved whiteout 
function…
(cf. <http://lists.gnu.org/archive/html/lilypond-user/2013-10/msg00578.html>)

Could \whiteout (with settable #’thickness or #’margin parameter) please use 
the grob outline, rather than just putting a white box behind the grog?

Thanks,
Kieren.