Work around a couple of minor display issues with max bridges set to 4:

Work around a couple of minor display issues with max bridges set to 4:

 - the highlighting for a set of 4 lines spilled outside the tile, so would
   leave white residue if undone;

 - the endpoints of sets of 4 lines were not completely overprinted by the
   circle of an island (at least on Windows), which was untidy.

Fixed by reducing the gap width for groups of lines which wouldn't otherwise
fit in a tile (only).

[originally from svn r6421]