[Session] Use total segments duration to check the period

[CastagnaIT]

Description

On the issue ISA its not able to switch to the right period when you try to do a video seek
manifest case: segtpl_multiperiod_ads.mpd.txt

from the manifest the ADS periods (marked with urn:scte:scte35:2013:xml) dont have the "duration" attribute but only "start" attrib.
with the parser we calculate the period duration by looking for the "start" attrib of the next period (by difference),
but if you calculate the duration, you can see that dont match with the total segments duration (of SegmentTemplate),
this seem to lead to in a segments hole in part of the period

It seems that Dash when periods are reported as ADS the period duration is not exact but estimated, but i have not yet found enough info to confirm this.

Then since there are parts of code that use "Period->GetDuration" lead to a wrong behaviour, as you can see on CSession::SeekTime try find the chapter on a wrong time

my idea is instead looking to each period duration, use total segments duration, that must be a safe value

todo:

1. need a feedback, i cant test it by myself atm since my only MPD multiperiod TV streaming is completely dead...
2. check for some regressions?
3. it would be useful to have confirmation from other streams that Dash period ADS have estimated and not precise durations

Motivation and context

attempt to fix #1500

How has this been tested?

NOT TESTED!

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Clean up (non-breaking change which removes non-working, unmaintained functionality)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that will cause existing functionality to change)
• Cosmetic change (non-breaking change that doesn't touch code)
• None of the above (please explain below)

Checklist:

• I have read the Contributing document
• My code follows the Code Guidelines of this project
• My change requires a change to the Wiki documentation
• I have updated the documentation accordingly