android: post_processor: Use CameraBuffer API

Use the newly introduced CameraBuffer class as the type for the
destination buffer in the PostProcessor class hierarchy in place of the
libcamera::MappedFrameBuffer one and use its API to retrieve the length
and the location of the CameraBuffer plane allocated for JPEG
post-processing.

Remove all the assumption on the underlying memory storage and only go
through the CameraBuffer API when dealing with memory buffers. To do so
rework the Encoder interface to use a raw pointer and an explicit size
to remove access to the Span<uint8_t> maps that serve as memory storage
for the current implementation but might not be ideal for other memory
backend.

Now that the whole PostProcessor hierarchy has been converted to use
the CameraBuffer API remove libcamera::MappedBuffer as base class
of the CameraBuffer interface and only reply on its interface.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>

src/android/mm/generic_camera_buffer.cpp

@@ -7,6 +7,7 @@
 
 #include "../camera_buffer.h"
 
+#include <libcamera/internal/buffer.h>
 #include "libcamera/internal/log.h"
 
 using namespace libcamera;