v4l2: Add split-field interlacing to v4l2src

Submitted by Zeeshan Ali @zeenix

Assigned to Nicolas Dufresne @ndufresne

Link to original bug (#797062)

Description

These patches add split-field (alternate) interlacing support to v4l2src and deinterlace elements.

These patches are not yet ready to be merged as the caps lack the needed feature, which will be added soon. The point of submitting them already was to already get opinions/suggestions on the main changes.

Also I'm not entirely sure if the (short-cut) approach I took for deinterlace is what we would want. Suggestions welcome.