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.cpp

@@ -57,6 +57,12 @@ bool AsyncShaderCompiler::HasPendingWork()
   return !m_pending_work.empty() || m_busy_workers.load() != 0;
 }
 
+bool AsyncShaderCompiler::HasCompletedWork()
+{
+  std::lock_guard<std::mutex> guard(m_completed_work_lock);
+  return !m_completed_work.empty();
+}
+
 void AsyncShaderCompiler::WaitUntilCompletion()
 {
   while (HasPendingWork())