Implement EFB copy filter and gamma in hardware backends

Also makes y_scale a dynamic parameter for EFB copies, as it doesn't
make sense to keep it as part of the uid, otherwise we're generating
redundant shaders.

Source/Core/VideoCommon/VideoConfig.h

@@ -73,6 +73,7 @@ struct VideoConfig final
   int iMaxAnisotropy;
   std::string sPostProcessingShader;
   bool bForceTrueColor;
+  bool bDisableCopyFilter;
 
   // Information
   bool bShowFPS;