Fix some cc608 field number handling.  Refs #11899.

More accurately decodes CEA-608 captions encoded in SCTE-20 user data.
  * Field number 3 is merged with field number 1 instead of 2.
  * Data in field number 3 is not rejected.
  * Field numbers 1 and 2 are then swapped if top_field_first is false.
These changes are based on the SCTE 20 handling code from ccextractor.
(cherry picked from commit e20a6310b1a2716ce601950a7278726c6d41b538)