mpeg2: add sequence display extension information

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Merge commit '8a06794112d02a5707f766b8b38a5d50dfd03898'

* commit '8a06794112d02a5707f766b8b38a5d50dfd03898':
  mpeg2: add sequence display extension information

Conflicts:
	libavcodec/mpeg12enc.c
	tests/ref/lavf/gxf
	tests/ref/lavf/mxf
	tests/ref/lavf/mxf_d10
	tests/ref/lavf/ts
	tests/ref/seek/lavf-gxf
	tests/ref/seek/lavf-mxf
	tests/ref/seek/lavf-ts
	tests/ref/seek/vsynth2-mpeg2-422
	tests/ref/seek/vsynth2-mpeg2-idct-int
	tests/ref/seek/vsynth2-mpeg2-ilace
	tests/ref/seek/vsynth2-mpeg2-ivlc-qprd
	tests/ref/seek/vsynth2-mpeg2-thread
	tests/ref/seek/vsynth2-mpeg2-thread-ivlc
	tests/ref/vsynth/vsynth1-mpeg2
	tests/ref/vsynth/vsynth1-mpeg2-422
	tests/ref/vsynth/vsynth1-mpeg2-idct-int
	tests/ref/vsynth/vsynth1-mpeg2-ilace
	tests/ref/vsynth/vsynth1-mpeg2-ivlc-qprd
	tests/ref/vsynth/vsynth1-mpeg2-thread
	tests/ref/vsynth/vsynth1-mpeg2-thread-ivlc
	tests/ref/vsynth/vsynth2-mpeg2
	tests/ref/vsynth/vsynth2-mpeg2-422
	tests/ref/vsynth/vsynth2-mpeg2-idct-int
	tests/ref/vsynth/vsynth2-mpeg2-ilace
	tests/ref/vsynth/vsynth2-mpeg2-ivlc-qprd
	tests/ref/vsynth/vsynth2-mpeg2-thread
	tests/ref/vsynth/vsynth2-mpeg2-thread-ivlc

Merged-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mpeg12enc: store sequence display extension only if we have something to store in it

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>