tracks: Scale thickness of "no track here" crosses

tracks: Scale thickness of "no track here" crosses

Simply replace the calls to draw_line with calls to draw_thick_line.

We need to choose a thickness parameter.  The preferred tile size is
30, and the "draw_line" function draws a 1-pixel line, so the
thickness right now is 1/30 the tile size, at the preferred size.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>