OGL: Re-implement async shader compiling

Source/Core/VideoBackends/Null/NullBackend.cpp

@@ -46,6 +46,7 @@ void VideoBackend::InitBackendInfo()
   g_Config.backend_info.bSupportsST3CTextures = false;
   g_Config.backend_info.bSupportsBPTCTextures = false;
   g_Config.backend_info.bSupportsFramebufferFetch = false;
+  g_Config.backend_info.bSupportsBackgroundCompiling = false;
 
   // aamodes: We only support 1 sample, so no MSAA
   g_Config.backend_info.Adapters.clear();