basetsmux: rework SCTE section handling to handle passthrough

basetsmux: rework SCTE section handling to handle passthrough

mpegtsmux can receive SCTE sections from two origins: events
created by the application, and events forwarded downstream by
mpegtsdemux, containing sections that may not have been fully
parsed, and additional data to help tsmux translate times to
the correct domain, both for requesting keyframes and calculating
an accurate pts_adjustment.

The complete approach is documented further in a comment above
the relevant function.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/913>