libcamera: pipeline_handler: Decouple buffer import and export

Use the V4L2 buffer orphaning feature, exposed through
V4L2VideoDevice::exportBuffers(), to decouple buffer import and export.
The PipelineHandler::importFrameBuffers() function is now called for all
streams regardless of whether exportFrameBuffers() has been called or
not. This simplifies the Camera implementation slightly, and opens the
door to additional simplifications.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

include/libcamera/camera.h

@@ -113,7 +113,6 @@ private:
 	friend class FrameBufferAllocator;
 	int exportFrameBuffers(Stream *stream,
 			       std::vector<std::unique_ptr<FrameBuffer>> *buffers);
-	int freeFrameBuffers(Stream *stream);
 	/* \todo Remove allocator_ from the exposed API */
 	FrameBufferAllocator *allocator_;
 };