VideoCommon: add method to async shader compiler to clear pending/completed work (used on shutdown), this will in turn clear up any resources that the worker items may have held onto

Source/Core/VideoCommon/ShaderCache.cpp

@@ -109,7 +109,10 @@ void ShaderCache::Shutdown()
   // This may leave shaders uncommitted to the cache, but it's better than blocking shutdown
   // until everything has finished compiling.
   if (m_async_shader_compiler)
+  {
     m_async_shader_compiler->StopWorkerThreads();
+    m_async_shader_compiler->ClearAllWork();
+  }
 
   ClosePipelineUIDCache();
 }