tracks: Roughly double the thickness of the "no track" crosses

tracks: Roughly double the thickness of the "no track" crosses

The default of 1/30 is rather thin, and probably wasn't chosen
deliberately (since it was just inherited from the default 1-pixel
line width, and the preferred tile size).

Thicker crosses stand out more and make play easier.

Use 1/16 since it's a rounder number than 1/15 :-).

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