Checking that the captured field corresponds to the last required field
depending on the requested field order before completing the buffer
isn't enough. When the first field at stream start corresponds to the
last required field, this would result in returning an interlaced buffer
containing a single field.

Fix this by keeping track of the fields captured in the buffer, and make
sure that both fields are present for alternate field orders.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Tested-by: Enrico Butera <ebutera@users.sourceforge.net>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>