adaptivedemux2: Rework input download wakeups

adaptivedemux2: Rework input download wakeups

Change the way streams are woken up to download more data.

Instead of checking the level on tracks that are being
output as data is dequeued, calculate a 'wakeup time'
at which it should download more data, and wake up
the stream when the global output position crosses
that threshold.

For efficiency, compute the earliest wakeup time
for all streams and store it on the period, so the
output loop can quickly check only a single value
to decide if something needs waking up.

Does the same buffering as the previous method,
but ensures that as we approach the end of
one period, the next period continues incrementally
downloading data so that it is fully buffered when
the period starts.

Fixes issues with multi-period VOD content where
download of the second period resumes only after
the first period is completely drained.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3055>