Guillaume Desmottes authored Aug 08, 2018 and Guillaume Desmottes committed Mar 25, 2020

When using this mode each frame is split in two fields, each one being
transferred using its own buffer.
This is implemented with the V4L2_FIELD_ALTERNATE field format in v4l2.

This mode is enabled using a caps filter such as
"v4l2src ! video/x-raw\(format:Interlaced\)"

Here are the main changes related to this feature:

- use the INTERLACED caps feature with this mode.

- in this mode both fields of a given frame have the same sequence/offset
so adjust the algorithm checking for lost field/frame accordingly.

- double pool's min number of buffers as each frame requires 2 buffers.

Fix #504
Co-authored-by: Zeeshan Ali <zeenix@collabora.co.uk>