Fix custom_entity:set_origin() not working as it should
Fix custom_entity:set_origin() not working as it should. Bug found by Realguigui in http://forum.solarus-games.org/index.php/topic,877.msg4882.html#msg4882 See the link above for more info.
According to what Christopho said, when we change the origin point of a custom entity, the position should change and the bounding box should be kept unchanged. However, what happens is the opposite (the bounding box coordinates change and the position does not). This gives problems when we try to synchronize the sprites position by using the function "custom_entity:set_origin(ox, oy)".