mpegtsmux: send multiple SCTE sections at once

Sometimes client wants to send multiple SCTE events at once, either from application or as pass-through

Current code

if (mux->pending_scte35_section)
      gst_mpegts_section_unref (mux->pending_scte35_section);

prevents this. Proposed changes save all SCTE into a linked list, then send one by one.

Perhaps sending with a g_list_iterate would take fewer lines but I wanted to preserve the logic of calling tsmux_send_section outside the lock.