mpeg2dec: drop B-frame on open gop

mpeg2dec: drop B-frame on open gop

Enhance open gop detection to drop B-frame which are invalid
before the first reference frame.

In stream such
gst-integration-testsuites/medias/defaults/mxf/op2b-mpeg2-wave_hd.mxf,
the two first frames must be dropped as we detect an open GOP situation
but in another media, such as http://col.la/1920X1080IXDCAMEX5MIN, the
first frames should not be dropped as we are in a closed GOP situation.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-ugly/-/merge_requests/84>