libcamera: object: Add deleteLater() support

This commit adds support to schedule the deletion of an Object to the
thread it is bound to (similar to [1]). An Object getting destroyed
by a different thread is considered as a violation as per the
libcamera threading model.

This will be useful for an Object where its ownership is shared via
shared pointers in different threads. If the thread which drops the
last reference of the Object is a different thread, the destructors
get called in that particular thread, not the one Object is bound to.
Hence, in order to resolve this kind of situation, the creation of
shared pointer can be accompanied by a custom deleter which in turns
use deleteLater() to ensure the Object is destroyed in its own thread.

[1] https://doc.qt.io/qt-5/qobject.html#deleteLater

Signed-off-by: Umang Jain <email@uajain.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

src/libcamera/message.cpp

@@ -49,6 +49,8 @@ std::atomic_uint Message::nextUserType_{ Message::UserMessage };
  * \brief Asynchronous method invocation across threads
  * \var Message::ThreadMoveMessage
  * \brief Object is being moved to a different thread
+ * \var Message::DeferredDelete
+ * \brief Object is scheduled for deletion
  * \var Message::UserMessage
  * \brief First value available for user-defined messages
  */