V4L/DVB (5289): Add support for video output overlays.

Add V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY support.
Also add support for local and global alpha overlays.
Add new field enums V4L2_FIELD_INTERLACED_TB and V4L2_FIELD_INTERLACED_BT.
These changes are needed to support the ivtv On Screen Display features.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>