Bug 0001789 correction (incorrect battle field detection)

There was 		
//look only for objects covering given tile
		if( !obj || obj->pos.z != tile.z
			|| !obj->coveringAt(tile.x - obj->pos.x, tile.y - obj->pos.y))

But in coveringAt there is 
bool CGObjectInstance::coveringAt(int x, int y) const
{
	return appearance.isVisibleAt(pos.x - x, pos.y - y);
}
So I changed it to
		if( !obj || obj->pos.z != tile.z
			|| !obj->coveringAt(tile.x, tile.y))
Seems to work (tested a little), but I don't yet understand mechanics of map layout. So I need proof from someone about coveringAt, that my correction is right.