VideoCommon: De-globalize PixelShaderManager class.

Source/Core/VideoCommon/VideoBackendBase.cpp

@@ -330,7 +330,7 @@ void VideoBackendBase::InitializeShared()
   VertexLoaderManager::Init();
   VertexShaderManager::Init();
   GeometryShaderManager::Init();
-  PixelShaderManager::Init();
+  system.GetPixelShaderManager().Init();
   TMEM::Init();
 
   g_Config.VerifyValidity();