video: Add GST_VIDEO_INTERLACE_MODE_ALTERNATE

video: Add GST_VIDEO_INTERLACE_MODE_ALTERNATE

Add a new interlace mode enum to represent buffers containing a single
field of an interlaced video in a buffer. The name is based on the
equivalent video format in the V4L2 API, V4L2_FIELD_ALTERNATE:

https://01.org/linuxgraphics/gfx-docs/drm/media/uapi/v4l/field-order.html

Since caps fields are optional, we also introduce a new caps feature,
"format:Interlaced" that always goes with "alternate" interlace mode to ensure
that caps for this incompatible format are incompatible with other interlaced
and progressive video caps.

https://bugzilla.gnome.org/show_bug.cgi?id=796106