gpu: Add shut down method to synchronize threads before destruction

src/video_core/gpu.cpp

@@ -312,6 +312,12 @@ struct GPU::Impl {
         cpu_context->MakeCurrent();
     }
 
+    void NotifyShutdown() {
+        std::unique_lock lk{sync_mutex};
+        shutting_down.store(true, std::memory_order::relaxed);
+        sync_cv.notify_all();
+    }
+
     /// Obtain the CPU Context
     void ObtainContext() {
         cpu_context->MakeCurrent();
@@ -859,6 +865,10 @@ void GPU::Start() {
     impl->Start();
 }
 
+void GPU::NotifyShutdown() {
+    impl->NotifyShutdown();
+}
+
 void GPU::ObtainContext() {
     impl->ObtainContext();
 }