Change the start number for $SubNumber$ template to match new DASH spec

[emilsas]

Have you read the FAQ and checked for duplicate open issues?
Yes

Is your feature request related to a problem? Please describe.
Yes, in new spec of DASH, the sub-number of the first Partial Segment in the sequence is 0, as stated in section 5.3.9.7.2 Hierarchical Templating and Sub-Numbering. This change in the spec is also apply to match compatibility with LL-HLS parts.

Describe the solution you'd like
Change the start number of the first Partial Segment.

Additional context
This enhancement will be needed for correct implementation of L3D-DASH.

Are you planning to send a PR to add it?
Yes