WIP: v4l2: Fix alternate interlacing caps and field values
Alternate interlacing was broken for v4l2sink element and had some issues in caps reporting. The caps negotiation was also broken in many ways.
ToDo:
- Ensure the following fails
v4l2src ! video/x-raw\(format:Interlaced\),interlaced-mode=progressive ! fakevideosink
- Ensure the following fails
v4l2src ! video/x-raw,interlaced-mode=alternate ! fakevideosink
- Investigate why all second fields are dropped with vivid
cc @gdesmott