Experimental UI tweak enabled by a hacky environment variable:

Experimental UI tweak enabled by a hacky environment variable:
suppress the display of `this square can't be a light' blobs in a
lit square, on the grounds that we already know _lit_ squares can't
be lights. This makes the solved game look cleaner (I've always
thought the detritus of blobs on some but not all non-light squares
looked messy), but on the other hand it's slightly jarring during
play. So I'm checking it in, but as a configurable option which is
off by default.

[originally from svn r7656]