ext/vorbis/vorbisenc.*: Set duration on encoded buffers. This allows oggmux's max_page_delay parameter to actually work.

Original commit message from CVS:
* ext/vorbis/vorbisenc.c: (gst_vorbisenc_setup),
(gst_vorbisenc_buffer_from_packet):
* ext/vorbis/vorbisenc.h:
Set duration on encoded buffers. This allows oggmux's
max_page_delay parameter to actually work.