The bones of what will become a Metal backend.

This isn't anywhere close to usable (not even a blank screen will work), let alone complete. It's about 20% done.

src/modules/graphics/metal/Shader.mm

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+/**
+ * Copyright (c) 2006-2020 LOVE Development Team
+ *
+ * This software is provided 'as-is', without any express or implied
+ * warranty.  In no event will the authors be held liable for any damages
+ * arising from the use of this software.
+ *
+ * Permission is granted to anyone to use this software for any purpose,
+ * including commercial applications, and to alter it and redistribute it
+ * freely, subject to the following restrictions:
+ *
+ * 1. The origin of this software must not be misrepresented; you must not
+ *    claim that you wrote the original software. If you use this software
+ *    in a product, an acknowledgment in the product documentation would be
+ *    appreciated but is not required.
+ * 2. Altered source versions must be plainly marked as such, and must not be
+ *    misrepresented as being the original software.
+ * 3. This notice may not be removed or altered from any source distribution.
+ **/
+
+#include "Shader.h"
+#include "Graphics.h"
+
+// glslang
+#include "libraries/glslang/glslang/Public/ShaderLang.h"
+#include "libraries/glslang/SPIRV/GlslangToSpv.h"
+#include "libraries/spirv_cross/spirv_msl.hpp"
+
+namespace love
+{
+namespace graphics
+{
+namespace metal
+{
+
+Shader::Shader(love::graphics::ShaderStage *vertex, love::graphics::ShaderStage *pixel)
+	: love::graphics::Shader(vertex, pixel)
+{ @autoreleasepool {
+	auto gfx = Graphics::getInstance();
+
+	using namespace glslang;
+	using namespace spirv_cross;
+
+	// TODO: can this be done in ShaderStage (no linking)?
+
+	glslang::TProgram program;
+
+	if (vertex != nullptr)
+		program.addShader((TShader *) vertex->getHandle());
+
+	if (pixel != nullptr)
+		program.addShader((TShader *) pixel->getHandle());
+
+	if (!program.link(EShMsgDefault))
+	{
+		//err = "Cannot compile shader:\n\n" + std::string(program.getInfoLog()) + "\n" + std::string(program.getInfoDebugLog());
+	}
+
+	for (int i = 0; i < EShLangCount; i++)
+	{
+		auto intermediate = program.getIntermediate((EShLanguage)i);
+		if (intermediate == nullptr)
+			continue;
+
+		spv::SpvBuildLogger logger;
+		glslang::SpvOptions opt;
+		opt.validate = true;
+
+		std::vector<unsigned int> spirv;
+
+		{
+//			timer::ScopedTimer("shader stage");
+			GlslangToSpv(*intermediate, spirv, &logger, &opt);
+		}
+
+		std::string msgs = logger.getAllMessages();
+//		printf("spirv length: %ld, messages:\n%s\n", spirv.size(), msgs.c_str());
+
+		// Compile to GLSL, ready to give to GL driver.
+		try
+		{
+
+//			printf("GLSL INPUT SOURCE:\n\n%s\n\n", pixel->getSource().c_str());
+
+			CompilerMSL msl(std::move(spirv));
+
+			CompilerMSL::Options options;
+
+#ifdef LOVE_IOS
+			options.platform = CompilerMSL::Options::iOS;
+#else
+			options.platform = CompilerMSL::Options::macOS;
+#endif
+
+			msl.set_msl_options(options);
+
+			std::string source = msl.compile();
+//			printf("MSL SOURCE:\n\n%s\n\n", source.c_str());
+
+			NSString *nssource = [[NSString alloc] initWithBytes:source.c_str()
+														  length:source.length()
+														encoding:NSUTF8StringEncoding];
+
+			NSError *err = nil;
+			id<MTLLibrary> library = [gfx->device newLibraryWithSource:nssource options:nil error:&err];
+
+		}
+		catch (std::exception &e)
+		{
+			printf("Error parsing SPIR-V shader source: %s\n", e.what());
+		}
+	}
+}}
+
+Shader::~Shader()
+{ @autoreleasepool {
+
+}}
+
+} // metal
+} // graphics
+} // love