The only thing that can be used in the chain function is the input segment. The
output segment might not be available at all yet or out of sync with the current
input segment.
Also because of that, the unadjusted timestamp has to be used for the
calculations as the adjustment is only part of the output segment.
This fixes the deadline calculation and the handling of force-keyunit events for
encoders using frame reordering (i.e. setting a minimum PTS).
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8842>
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