Rewrite host GPU abstraction

- Don't have to repeat the same thing for 4 renderers.
 - Add native Metal renderer.

src/util/spirv_compiler.h

@@ -0,0 +1,47 @@
+// SPDX-FileCopyrightText: 2023 Connor McLaughlin <stenzek@gmail.com>
+// SPDX-License-Identifier: (GPL-3.0 OR CC-BY-NC-ND-4.0)
+
+#pragma once
+
+#include "common/types.h"
+
+#include "gsl/span"
+
+#include <optional>
+#include <string>
+#include <string_view>
+#include <vector>
+
+enum class GPUShaderStage : u8;
+
+namespace SPIRVCompiler {
+
+enum CompileOptions
+{
+  DebugInfo = (1 << 0),
+  VulkanRules = (1 << 1),
+};
+
+// SPIR-V compiled code type
+using SPIRVCodeType = u32;
+using SPIRVCodeVector = std::vector<SPIRVCodeType>;
+
+// Compile a vertex shader to SPIR-V.
+std::optional<SPIRVCodeVector> CompileVertexShader(std::string_view source_code, u32 options);
+
+// Compile a fragment shader to SPIR-V.
+std::optional<SPIRVCodeVector> CompileFragmentShader(std::string_view source_code, u32 options);
+
+// Compile a compute shader to SPIR-V.
+std::optional<SPIRVCodeVector> CompileComputeShader(std::string_view source_code, u32 options);
+
+std::optional<SPIRVCodeVector> CompileShader(GPUShaderStage stage, std::string_view source_code, u32 options);
+
+#ifdef __APPLE__
+
+// Converts a SPIR-V shader into MSL.
+std::optional<std::string> CompileSPIRVToMSL(gsl::span<const SPIRVCodeType> spv);
+
+#endif
+
+} // namespace SPIRVCompiler