Fix antialiasing artifacts around edges of Bridges islands.

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.

[originally from svn r10282]