Refactor p_pose and related variables

Refactor p_pose and related variables

p_pose was used to store position of the tracked object. Its use was
confusing, because only the center point was updates and width/height
not. Width/height was updated only in getBBox() according to
p_current_scale.

This commit renames p_pose to p_init_pose and does not touch it after
init(). We store the position of the tracked object in
p_current_scale. getBBox is updated to calculate the bounding box from
p_current_center, p_current_scale and p_init_pose.