Implement reverse timestamp calculation for Opus (fixes calculated
timestamps on first page), and correct for pre-skip and end trimming.

Signed-off-by: Ralph Giles <giles@mozilla.com>

Reviewer note: Pretty sure we can make queued_duration overflow
by feeding it a long stream of pages without proper granulepos.
I decided not to worry about it; the caller will get unexpected
nonesense, but the stream is also invalid. Just wanted to note
the issue for future reference.