Replace time-windowed bitrate management setup with a one-pass

Replace time-windowed bitrate management setup with a one-pass
'greedy' bit reservoir system.

Advantages:

no additional encoder latency
more predictable buffering
lower memory usage

These changes introduce two new vorbis_encode_ctls to get to the new
functionality.  The old ectl calls are still supported for
compatability and binary library compatability is not broken (but
libvorbis/file/enc need to be upgraded as a set).

...now going to update oggenc.

Monty

svn path=/trunk/vorbis/; revision=5726