v4l2object: Read driver selected interlace mode

Submitted by Nicolas Dufresne @ndufresne

Link to original bug (#797306)

Description

This is to fix a bug reported by Tim Harvey on the mailing list. His
original message was:

I've got a v4l2src capture pipeline using an interlaced video source
which used to work on GStreamer 1.8.3 that I've found no longer
negotiates on GStreamer 1.14.1:

root@bionic-armhf:# v4l2-ctl -d7 --get-fmt-video
Format Video Capture:
Width/Height : 720/480
Pixel Format : 'UYVY' (UYVY 4:2:2)
Field : Sequential Bottom-Top
Bytes per Line : 1440
Size Image : 691200
Colorspace : SMPTE 170M
Transfer Function : Rec. 709
YCbCr/HSV Encoding: ITU-R 601
Quantization : Limited Range
Flags :
root@bionic-armhf:# gst-launch-1.0 --version
gst-launch-1.0 version 1.14.1
GStreamer 1.14.1
https://launchpad.net/distros/ubuntu/+source/gstreamer1.0
root@bionic-armhf:~# gst-launch-1.0 v4l2src device=/dev/video7 !
video/x-raw,format=UYVY ! jpegenc ! rtpjpegpay ! udpsink host=$SERVER
port=5000
Setting pipeline to PAUSED ...
Pipeline is live and does not need PREROLL ...
Setting pipeline to PLAYING ...
New clock: GstSystemClock
ERROR: from element /GstPipeline:pipeline0/GstV4l2Src:v4l2src0: Device
'/dev/video7' does not support progressive interlacing
Additional debug info:
gstv4l2object.c(3768): gst_v4l2_object_set_format_full ():
/GstPipeline:pipeline0/GstV4l2Src:v4l2src0:
Device wants interleaved interlacing
Execution ended after 0:00:00.003064667
Setting pipeline to PAUSED ...
Setting pipeline to READY ...
Setting pipeline to NULL ...
Freeing pipeline ...

Any idea what changed here that causes this?