libcamera: Refactor the camera configuration storage and API

Refactor the CameraConfiguration structure to not rely on Stream
instances. This is a step towards making the camera configuration object
more powerful with configuration validation using "try" semantics.

The CameraConfiguration now exposes a simple vector-like API to access
the contained stream configurations. Both operator[]() and at() are
provided to access elements. The isEmpty() method is renamed to empty()
and the methods reordered to match the std::vector class.

As applications need access to the Stream instances associated with the
configuration entries in order to associate buffers with streams when
creating requests, expose the stream selected by the pipeline handler
through a new StreamConfiguration::stream().

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

src/libcamera/stream.cpp

@@ -58,6 +58,28 @@ namespace libcamera {
  * \brief Requested number of buffers to allocate for the stream
  */
 
+/**
+ * \fn StreamConfiguration::stream()
+ * \brief Retrieve the stream associated with the configuration
+ *
+ * When a camera is configured with Camera::configure() Stream instances are
+ * associated with each stream configuration entry. This method retrieves the
+ * associated Stream, which remains valid until the next call to
+ * Camera::configure() or Camera::release().
+ *
+ * \return The stream associated with the configuration
+ */
+
+/**
+ * \fn StreamConfiguration::setStream()
+ * \brief Associate a stream with a configuration
+ *
+ * This method is meant for the PipelineHandler::configure() method and shall
+ * not be called by applications.
+ *
+ * \param[in] stream The stream
+ */
+
 /**
  * \brief Assemble and return a string describing the configuration
  *