libcamera: allocator: Add FrameBufferAllocator to help applications allocate buffers

The FrameBuffer interface is based on the idea that all buffers are
allocated externally to libcamera and are only used by it. This is meant
to create a simpler API centered around usage of buffers, regardless of
where they come from.

Linux however lacks a centralized allocator at the moment, and not all
users of libcamera are expected to use another device that could provide
suitable buffers for the camera. This patch thus adds a helper class to
allocate buffers internally in libcamera, in a way that matches the
needs of the FrameBuffer-based API.

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

include/libcamera/camera.h

@@ -20,6 +20,7 @@
 namespace libcamera {
 
 class Buffer;
+class FrameBufferAllocator;
 class PipelineHandler;
 class Request;
 
@@ -126,6 +127,10 @@ private:
 
 	bool disconnected_;
 	State state_;
+
+	/* Needed to update allocator_ and to read state_ and activeStreams_. */
+	friend class FrameBufferAllocator;
+	FrameBufferAllocator *allocator_;
 };
 
 } /* namespace libcamera */