Well-defined screen geometry before init and after shutdown

a) initialize termw and termh to -1.

b) make an API guaranty that the screen dimensions are "negative" (no
need to say -1) if tb_init() has not been called or if tb_shutdown()
has been called without a subsequent tb_init() (i.e., if we're in tb
screen mode).

c) in tb_shutdown(), set termw/termh back to -1.