COMMENT: Check whether in the air
if relative position (0,1) is collision free for Only solid objects
      set the gravity to 0.5 in direction 270
else
      set the gravity to 0 in direction 270
COMMENT: Limit the vertical speed
if vspeed is larger than 12
      set variable vspeed to 12
COMMENT: Check whether on a ladder
if at relative position (0,0) there is object obj_ladder
      set the gravity to 0 in direction 270
      set variable vspeed to 0
      set the sprite to spr_climbing with subimage 0 and speed 1