Fix BTL_initSkillChange's array traversal

BTL_initSkillChange was created with the assumption that the iterator variable created in a for loop was modifiable. For performance reasons, SRB2's lua engine does not allow this, which would cause strange quirks whenever a skill change would swap a passiveskill to a normal skill and vice versa. This can be fixed by changing the for loop into a while loop, and handling the iterator variable manually, allowing for traversal of a dynamically shrinking array.