video: No support for field-by-field interlaced mode

Submitted by Zhu, Haiyang

Link to original bug (#756840)

Description

My command:

gst-launch-1.0 filesrc location=interlace.yuy2! videoparse format=yuy2 width=1280 height=720 interlaced=true ! vaapipostproc deinterlace-mode=interlaced deinterlace-method=bob format=nv12 ! vaapisink.

The interlace.yuy2 is interlaced RAW file, which contains top and bottom frames alternately.

The expect results:
vaapipostproc could deinterlace video and shown by vaapisink.

The real results:
The shown video is not deinterlaced.