Fix antialiasing artifacts around edges of Bridges islands.

When an island changes colour (because it becomes locked or
highlighted, or goes back to plain COL_FOREGROUND from one of those
states), we were just redrawing a filled circle over the previous one,
which only really worked before antialiasing was introduced.

Fixed by reworking the entire drawing edifice to be grid-square based,
so that every grid square is either redrawn in full or left alone.
Grid squares come in two types - island and bridge - but a bridge
square can have neighbouring islands overlap into it, and the bridges
entering an island overlap the island square too, so we end up with
quite a large collection of bitfields describing each square's
contents, and some care must be taken to draw each square in the right
order.


git-svn-id: svn://svn.tartarus.org/sgt@10282 cda61777-01e9-0310-a592-d414129be87e