libcamera: request: remove prepare()

The association of buffers to a request can be done directly in
addBuffer() instead of when the request is queued to the camera. Keep
the check that a request contains buffers by moving it to
Camera::queueRequest() where prepare() was previously called.

As a bonus we can remove a friend statement in Request.

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

include/libcamera/request.h

@@ -48,10 +48,8 @@ public:
 	bool hasPendingBuffers() const { return !pending_.empty(); }
 
 private:
-	friend class Camera;
 	friend class PipelineHandler;
 
-	int prepare();
 	void complete();
 
 	bool completeBuffer(Buffer *buffer);