dashdemux: skip period with same start time

Submitted by Siva Vedula

Description

There is a redundant period present while using external mpd. Start time of an external mpd period and start time of regular mpd period computed to same. Due to this conflict, error is returned and "Invalid Manifest" error is thrown.

For example:

P1 period with start time 'x' and duration 60S in regular mpd,
next there is an external mpd period with two periods P2 and P3 of duration each 60S,
after this there is a period P4 with start time of 'x+120' in regular mpd.

For this, P3 start time is computed to 'x+120' and it is same as P4 start time. Due to same start time for two periods, in period start time validation check, error occurred.
This is observed in the below mpd:

http://vm2.dashif.org/livesim-dev/periods_60/xlink_30/insertad_2/testpic_2s/Manifest.mpd