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.h

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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.
+ **/
+
+#pragma once
+
+#include "graphics/Shader.h"
+#include "graphics/Graphics.h"
+#include "Metal.h"
+
+namespace love
+{
+namespace graphics
+{
+namespace metal
+{
+
+class Shader final : public love::graphics::Shader
+{
+public:
+
+	Shader(love::graphics::ShaderStage *vertex, love::graphics::ShaderStage *pixel);
+	virtual ~Shader();
+
+	// Implements Shader.
+	void attach() override;
+	std::string getWarnings() const override;
+	int getVertexAttributeIndex(const std::string &name) override;
+	const UniformInfo *getUniformInfo(const std::string &name) const override;
+	const UniformInfo *getUniformInfo(BuiltinUniform builtin) const override;
+	void updateUniform(const UniformInfo *info, int count) override;
+	void sendTextures(const UniformInfo *info, Texture **textures, int count) override;
+	bool hasUniform(const std::string &name) const override;
+	ptrdiff_t getHandle() const override;
+	void setVideoTextures(Texture *ytexture, Texture *cbtexture, Texture *crtexture) override;
+
+private:
+
+	id<MTLLibrary> library;
+
+}; // Metal
+
+extern Metal metal;
+
+} // metal
+} // graphics
+} // love