libcamera: pipeline_handler: Add camera disconnection support

Pipeline handlers are responsible for creating camera instances, but
also for destroying them when devices are unplugged. As camera objects
are reference-counted this isn't a straightforward operation and
involves the camera manager and camera object itself. Add two helper
methods in the PipelineHandler base class to register a camera and to
register a media device with the pipeline handler.

When registering a camera, the registerCamera() helper method will add
it to the camera manager. When registering a media device, the
registerMediaDevice() helper method will listen to device disconnection
events, and disconnect all cameras created by the pipeline handler as a
response.

Under the hood the PipelineHandler class needs to keep track of
registered cameras in order to handle disconnection. They can't be
stored as shared pointers as this would create a circular dependency
(the Camera class owns a shared pointer to the pipeline handler). Store
them as weak pointers instead. This is safe as a reference to the camera
is stored in the camera manager, and doesn't get removed until the
camera is unregistered from the manager by the PipelineHandler.

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

src/libcamera/pipeline/vimc.cpp

@@ -6,7 +6,6 @@
  */
 
 #include <libcamera/camera.h>
-#include <libcamera/camera_manager.h>
 
 #include "device_enumerator.h"
 #include "media_device.h"
@@ -58,7 +57,7 @@ bool PipeHandlerVimc::match(DeviceEnumerator *enumerator)
 	dev_->acquire();
 
 	std::shared_ptr<Camera> camera = Camera::create(this, "Dummy VIMC Camera");
-	manager_->addCamera(std::move(camera));
+	registerCamera(std::move(camera));
 
 	return true;
 }