GPU: Add scaling shader support

Currently only Bilinear (Sharp).

src/core/gpu_shadergen.h

@@ -0,0 +1,20 @@
+// SPDX-FileCopyrightText: 2019-2023 Connor McLaughlin <stenzek@gmail.com>
+// SPDX-License-Identifier: GPL-3.0
+
+#pragma once
+
+#include "util/shadergen.h"
+
+class GPUShaderGen : public ShaderGen
+{
+public:
+  GPUShaderGen(RenderAPI render_api, bool supports_dual_source_blend);
+  ~GPUShaderGen();
+
+  std::string GenerateDisplayVertexShader();
+  std::string GenerateDisplayFragmentShader();
+  std::string GenerateDisplaySharpBilinearFragmentShader();
+
+private:
+  void WriteDisplayUniformBuffer(std::stringstream& ss);
+};