to maintain the `visible' array accurately and hence actually switch
it on. This prevents us having to redraw the entire playing area on
any move, which means really big grids are now sensibly playable
without display lag.
[originally from svn r4221]
feature to arrange a mechanism that allows you to draw a whole
rectangle at a time by dragging rather than having to click each
edge individually.
[originally from svn r4209]
addition to the `game_state'. The new structure is intended to
contain ephemeral data pertaining to the game's user interface
rather than the actual game: things stored in the UI structure are
not restored in an Undo, for example.
make_move() is passed the UI to modify as it wishes; it is now
allowed to return the _same_ game_state it was passed, to indicate
that although no move has been made there has been a UI operation
requiring a redraw.
[originally from svn r4207]
