tracks: thicken the lines of the grid itself.

Since these lines are always orthogonal, it's easier to draw them
using draw_rect than draw_thick_line.

Previously, the grid lines were drawn just inside the top and left
edges of the region redrawn by draw_square(), so only the bottom and
right edges of the whole grid were not covered by any draw_square
call. To avoid having to shift the logical grid centre, I'm now
drawing parts of the grid outline on all four sides of the
draw_square() region, so that half the thickened grid edge protrudes
on every side of the grid as a whole.

In the process of splitting up the grid line width into the part on
the top and left edges and the part on the bottom and right, I've
renamed the confusingly named BORDER_WIDTH define (which wasn't used
anyway) to a set of things that make it clear that they're referring
to the grid lines as opposed to the border of the whole puzzle.

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