v4l2codecs: Rework handling of queues and pending requests

v4l2codecs: Rework handling of queues and pending requests

Starting from this patch, all queue and dequeue operation happening
on V4L2 is now abstracted with the request. Buffers are dequeued
automatically when pending requests are marked done and only 1 in-flight
request is now used.

Along with fixing issues with request not being reused with slice
decoders, this change reduces the memory footprint by allocating only
two bitstream buffers.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1881>