ShaderCache: Implement background shader compilation

This enables shaders to be compiled while the game is starting, instead
of blocking startup. If a shader is needed before it is compiled,
emulation will block.

Source/Core/VideoCommon/VideoConfig.h

@@ -169,6 +169,7 @@ struct VideoConfig final
   int iCommandBufferExecuteInterval;
 
   // Shader compilation settings.
+  bool bWaitForShadersBeforeStarting;
   UberShaderMode iUberShaderMode;
 
   // Number of shader compiler threads.