ShaderCache: Implement compile priority

Currently, when immediately compile shaders is not enabled, the
ubershaders will be placed before any specialized shaders in the compile
queue in hybrid ubershaders mode. This means that Dolphin could
potentially use the ubershaders for a longer time than it would have if
we blocked startup until all shaders were compiled, leading to a drop in
performance.

Source/Core/VideoCommon/AsyncShaderCompiler.h

@@ -8,6 +8,7 @@
 #include <condition_variable>
 #include <deque>
 #include <functional>
+#include <map>
 #include <memory>
 #include <mutex>
 #include <thread>
@@ -42,7 +43,9 @@ public:
     return std::make_unique<T>(std::forward<Params>(params)...);
   }
 
-  void QueueWorkItem(WorkItemPtr item);
+  // Queues a new work item to the compiler threads. The lower the priority, the sooner
+  // this work item will be compiled, relative to the other work items.
+  void QueueWorkItem(WorkItemPtr item, u32 priority);
   void RetrieveWorkItems();
   bool HasPendingWork();
   bool HasCompletedWork();
@@ -74,7 +77,9 @@ private:
   std::vector<std::thread> m_worker_threads;
   std::atomic_bool m_worker_thread_start_result{false};
 
-  std::deque<WorkItemPtr> m_pending_work;
+  // A multimap is used to store the work items. We can't use a priority_queue here, because
+  // there's no way to obtain a non-const reference, which we need for the unique_ptr.
+  std::multimap<u32, WorkItemPtr> m_pending_work;
   std::mutex m_pending_work_lock;
   std::condition_variable m_worker_thread_wake;
   std::atomic_size_t m_busy_workers{0};