adaptivedemux: Properly handle presentationTimeOffset for seeking and multi-period...

adaptivedemux: Properly handle presentationTimeOffset for seeking and multi-period streams

Segment start/time/position/base should only be modified if this is the first
time we send a segment, otherwise we will override values from the seek
segment if new streams have to be exposed as part of the seek.

Segment base should be calculated from the segment start based on the stream's
own segment, not the demuxer's segment. Both might differ slightly because of
the presentationTimeOffset.

Always add the presentationTimeOffset (relative to the period start, not
timestamp 0) to the segment start after resetting the stream's segment based
on the demuxer's segment (i.e. after seeks or stream restart). Also make sure
to keep the stream's segment up to date and not just send a new segment event
without storing the segment in the stream.

https://bugzilla.gnome.org/show_bug.cgi?id=745455