OGL: Use struct for post-processing shader options

This removes the need for token pasting, which isn't supported in GLSL
ES. Shouldn't cause any issues unless people are using reserved keywords
as option names.

Source/Core/VideoBackends/OGL/PostProcessing.h

@@ -38,7 +38,7 @@ private:
   std::unordered_map<std::string, GLuint> m_uniform_bindings;
 
   void CreateHeader();
-  std::string LoadShaderOptions(const std::string& code);
+  std::string LoadShaderOptions();
 };
 
 }  // namespace