oggmux: refactor how EOS is determined

oggmux: refactor how EOS is determined

This decreases the number of buffers held on each pad by one,
eliminating next_buffer.  Simplifies the logic by relying solely
on CollectPads to let us know when a pad is in EOS.  As a side
benefit, the collect pads related code is structured more like
other CollectPad users.

The previous code would occasionally mark the wrong pad as EOS,
causing the code to get in a state where all the streams were
finished, but EOS hadn't been sent to the source pad.