SCC writer cannot handle events with a media time of `0s`

[palemieux]

If the document contains an event at t=0s, then the SCC writer will not be able to create a pop-on event at 00:00:00:00 since the contents of the event must be loaded before 00:00:00:00 and negative times are not allowed by SCC. This is typically remediated by setting the start time of SCC files to 01:00:00:00.

Propose to add a start time offset option to SCC writer, e.g., "start_offset": "01:00:00:00".