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/VideoConfig.cpp

@@ -198,7 +198,7 @@ u32 VideoConfig::GetShaderCompilerThreads() const
 u32 VideoConfig::GetShaderPrecompilerThreads() const
 {
   // When using background compilation, always keep the same thread count.
-  if (bWaitForShadersBeforeStarting)
+  if (!bWaitForShadersBeforeStarting)
     return GetShaderCompilerThreads();
 
   if (!backend_info.bSupportsBackgroundCompiling)