Improve shadow bold mode: set the default shadow bold offset to +1

Improve shadow bold mode: set the default shadow bold offset to +1
not -1 (it turns out _most_ X fonts prefer the former, though
irritatingly my favourite real X font used to prefer the latter
which was why I made the X version of my Font Of Choice do so too),
and also clip to the boundaries of the rectangle we should be
drawing text in. This still doesn't completely prevent display
corruption in the case where text drawn in one sweep is partially
overwritten in a future one, but gnome-terminal has this problem
too, and now we've got the right default SB offset _and_ offer the
opportunity to reconfigure it I think this is pretty good for now.

[originally from svn r2184]