straight_movement:set_max_distance() bug

[llamaizing]

When using a straight movement, the calculation for distance the movement has traveled seems to always be from coordinates (0,0) when the distance of the movement should be calculated from whatever the coordinates of the object the movement is applied to were at the start of the movement.

Discussed here (with example code): http://forum.solarus-games.org/index.php/topic,1030.0.html