fixed bug with leaving steel grid after passing it in an irregular way

fixed bug with leaving steel grid after passing it in an irregular way

When using DF style reflecting steel walls together with MM style
fixed steel grid, it is possible to point the laser towards the grid
in a non 90° angle and still passing through it (with the laser beam
being reflected inside the grid until it leaves the grid on the other
side). However, this did not work when passing the steel grid from
bottom to top (upwards) into an "empty space" element.

This change fixes handling this special case.