scm/stencil.scm: redefine the whiteout function
so that it approximates the stencil's outline by
creating multiple displaced copies of the stencil.
Rename the previous whiteout function 'whiteout-box'.
scm/define-grob-properties.scm: define two
properties to go with the two whiteout functions.
lily/grob.cc: use the new whiteout functions and
properties.
scm/define-markup-commands.scm: define markup
commands to go with the two whiteout functions.