[media] vivid: add teletext support to VBI capture

This is useful to test teletext capture applications like alevt and mtt.

It also fixes a previously undetected bug where the PAL VBI start line
of the second field was off by one. Using the new field start defines
helps a lot fixing such bugs.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>