640x400 crashes with the largest screen size

[BSzili]

There is a rounding error in BorderSetView that causes the calculated screen size off by one for 640x400 when the full screen sizes (with the minimal or no hud) are used. This line calculates 401 for y2 instead of 400 which causes a crash:
https://github.com/jonof/jfsw/blob/master/src/border.c#L441
Clamping could be added after it to work around the problem, e.g. if (y2 > ydim) y2 = ydim;