Issue 4418/2 add and use new whiteout function

Issue 4418/2 add and use new whiteout function

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.