ShaderCache: Fix several issues in background shader compiling

- In D3D, shaders could be compiled on the main thread, blocking
startup.
- Reduced the latency between a pipeline being requested and used in all
backends in hybrid ubershader mode, when no shader stages were present.
- Fixed a case where async compilation could cause the same UID to be
appended multiple times to the UID cache.
- Fix incorrect number of threads being used when immediately compile
shaders was enabled.

Source/Core/VideoCommon/AsyncShaderCompiler.h

@@ -45,6 +45,7 @@ public:
   void QueueWorkItem(WorkItemPtr item);
   void RetrieveWorkItems();
   bool HasPendingWork();
+  bool HasCompletedWork();
 
   // Simpler version without progress updates.
   void WaitUntilCompletion();