android: post_processor: Make post processing async

Introduce a dedicated worker class derived from libcamera::Thread.
The worker class maintains a queue for post-processing requests
and waits for a post-processing request to become available.
It will process them as per FIFO before de-queuing it from the
queue.

The entire post-processing handling iteration is locked under
streamsProcessMutex_ which helps us to queue all the post-processing
request at once, before any of the post-processing completion slot
(streamProcessingComplete()) is allowed to run for post-processing
requests completing in parallel. This helps us to manage both
synchronous and asynchronous errors encountered during the entire
post processing operation. Since a post-processing operation can
even complete after CameraDevice::requestComplete() has returned,
we need to check and complete the descriptor from
streamProcessingComplete() running in the PostProcessorWorker's
thread.

This patch also implements a flush() for the PostProcessorWorker
class which is responsible to purge post-processing requests
queued up while a camera is stopping/flushing. It is hooked with
CameraStream::flush(), which isn't used currently but will be
used when we handle flush/stop scenarios in greater detail
subsequently (in a different patchset).

The libcamera request completion handler CameraDevice::requestComplete()
assumes that the request that has just completed is at the front of the
queue. Now that the post-processor runs asynchronously, this isn't true
anymore, a request being post-processed will stay in the queue and a new
libcamera request may complete. Remove that assumption, and use the
request cookie to obtain the Camera3RequestDescriptor.

Signed-off-by: Umang Jain <umang.jain@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

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