FrontendCommon: Add a post processing implementation

src/duckstation-libretro/libretro_host_display.h

@@ -1,6 +1,5 @@
 #pragma once
 #include "core/host_display.h"
-#include <memory>
 
 class LibretroHostDisplay final : public HostDisplay
 {
@@ -26,6 +25,8 @@ public:
   void ResizeRenderWindow(s32 new_window_width, s32 new_window_height) override;
   void DestroyRenderSurface() override;
 
+  bool SetPostProcessingChain(const std::string_view& config) override;
+
   bool CreateResources() override;
   void DestroyResources() override;