libcamera: thread: Support dispatching messages to main thread

Threads contain message queues and dispatch queued messages in their
event loop, in Thread::exec(). This mechanism prevents the main thread
from dispatching messages as it doesn't run Thread::exec().

Fix this by moving message dispatching from Thread::exec() to
EventDispatcher::processEvents().

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

src/libcamera/include/thread.h

@@ -43,6 +43,8 @@ public:
 	EventDispatcher *eventDispatcher();
 	void setEventDispatcher(std::unique_ptr<EventDispatcher> dispatcher);
 
+	void dispatchMessages();
+
 protected:
 	int exec();
 	virtual void run();
@@ -53,7 +55,6 @@ private:
 
 	void postMessage(std::unique_ptr<Message> msg, Object *receiver);
 	void removeMessages(Object *receiver);
-	void dispatchMessages();
 
 	friend class Object;
 	friend class ThreadData;