media: rpivid: Add a Pass0 to accumulate slices and rework job finish

Due to overheads in assembling controls and requests it is worth having
the slice assembly phase separate from the h/w pass1 processing. Create
a queue to service pass1 rather than have the pass1 finished callback
trigger the next slice job.

This requires a rework of the logic that splits up the buffer and
request done events. This code contains two ways of doing that, we use
Ezequiel Garcias <ezequiel@collabora.com> solution, but expect that
in the future this will be handled by the framework in a cleaner manner.

Fix up the handling of some of the memory exhaustion crashes uncovered
in the process of writing this code.

Signed-off-by: John Cox <jc@kynesim.co.uk>