libcamera: pipeline_handler: Register requests

Provide a call allowing requests to be registered and associated with
the pipeline handler after being constructed by the camera.

This provides an opportunity for the PipelineHandler to connect any
signals it may be interested in receiving for the request such as
getting notifications when the request is ready for processing when
using a fence.

While here, update the existing usage of the d pointer in
Camera::createRequest() to match the style of other functions.

Bug: https://github.com/raspberrypi/libcamera-apps/issues/217
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

include/libcamera/internal/pipeline_handler.h

@@ -59,6 +59,7 @@ public:
 	void stop(Camera *camera);
 	bool hasPendingRequests(const Camera *camera) const;
 
+	void registerRequest(Request *request);
 	void queueRequest(Request *request);
 
 	bool completeBuffer(Request *request, FrameBuffer *buffer);