Per https://dashif.org/docs/DASH-IF-IOP-v4.3.pdf 4.4.3.6 Signalling the last segment number in Period, there are three ways to signal the last segment number:
a. Use the lmsg signalling as defined in clause 4.4.3.5.
b. Use the Segment Timeline with @r value greater or equal to 0.
c. Add a Supplemental Descriptor with @schemeIdUri set to http://dashif.org/guidelines/last-segment-number with the @value set to the last segment number.
We do not support (a). This change adds support for (c) when Segment Timeline (b) is not used, i.e. when Representation has constant duration (could be approximate).
Under flag --dash_add_last_segment_number_when_needed (disabled by default).
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