Previously Rat.draw() hid any rat whose center fell inside a tunnel.
The clipping helper only handled single-entrance cells and returned None
for internal passages/crossroads, causing rats to vanish entirely.
Now internal tunnel cells (those with 0 or 2+ open sides) draw the rat
normally so it remains visible while walking through the tunnel.
Single-entrance cells keep the partial clip effect.
Also fix the visible-ratio math for DOWN/RIGHT clipping: the old
formulas subtracted cell_size from a local coordinate, producing
zero or negative visibility.
enne2
5afdf3705b