GPU: Add scaling shader support

Currently only Bilinear (Sharp).

src/util/shadergen.h

@@ -20,8 +20,6 @@ public:
   std::string GenerateUVQuadVertexShader();
   std::string GenerateFillFragmentShader();
   std::string GenerateCopyFragmentShader();
-  std::string GenerateDisplayVertexShader();
-  std::string GenerateDisplayFragmentShader(bool set_alpha_to_one = false);
 
   std::string GenerateImGuiVertexShader();
   std::string GenerateImGuiFragmentShader();