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/ShaderCache.h

@@ -72,7 +72,7 @@ private:
   void CompileMissingPipelines();
   void InvalidateCachedPipelines();
   void ClearPipelineCaches();
-  void PrecompileUberShaders();
+  void QueueUberShaderPipelines();
 
   // GX shader compiler methods
   std::unique_ptr<AbstractShader> CompileVertexShader(const VertexShaderUid& uid) const;