codecs: h264decoder: Add support for interlaced stream

codecs: h264decoder: Add support for interlaced stream

Initial support for interlaced stream. Subclass should implement
new_field_picture() vfunc. Otherwise, baseclass will assume that
subclass doesn't support interlaced stream.

Restrictions:
* Reference picture modification process for interlaced stream
  is not implemented yet
* PAFF (Picture Adaptive Frame Field) is not properly implemented.
* Field display ordering (e.g., top-field-first) decision should
  be enhanced via picture timing SEI parsing
* Gap in field picture should be handled

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1534>