Core/Movie: Add ability to run code in Host context

EndPlayInput runs on the CPU thread so it can't directly call
UpdateWantDeterminism. PlayController also tries to ChangeDisc
from the CPU Thread which is also invalid. It now just pauses
execution and posts a request to the Host to fix it instead.

The Core itself also did dodgy things like PauseAndLock-ing
from the CPU Thread and SetState from EmuThread which have been
removed.

Source/Core/Core/Core.h

@@ -11,6 +11,7 @@
 
 #pragma once
 
+#include <functional>
 #include <string>
 #include <vector>
 
@@ -91,4 +92,19 @@ void SetOnStoppedCallback(StoppedCallbackFunc callback);
 // Run on the Host thread when the factors change. [NOT THREADSAFE]
 void UpdateWantDeterminism(bool initial = false);
 
+// Queue an arbitrary function to asynchronously run once on the Host thread later.
+// Threadsafe. Can be called by any thread, including the Host itself.
+// Jobs will be executed in RELATIVE order. If you queue 2 jobs from the same thread
+// then they will be executed in the order they were queued; however, there is no
+// global order guarantee across threads - jobs from other threads may execute in
+// between.
+// NOTE: Make sure the jobs check the global state instead of assuming everything is
+//   still the same as when the job was queued.
+// NOTE: Jobs that are not set to run during stop will be discarded instead.
+void QueueHostJob(std::function<void()> job, bool run_during_stop = false);
+
+// Should be called periodically by the Host to run pending jobs.
+// WM_USER_JOB_DISPATCH will be sent when something is added to the queue.
+void HostDispatchJobs();
+
 }  // namespace