Add Metal backend (#1287)

.gitmodules

@@ -18,6 +18,10 @@
 	path = dependencies/imgui
 	url = https://github.com/ocornut/imgui
 	shallow = true
+[submodule "dependencies/metal-cpp"]
+	path = dependencies/metal-cpp
+	url = https://github.com/bkaradzic/metal-cpp.git
+	shallow = true
 [submodule "dependencies/xbyak_aarch64"]
 	path = dependencies/xbyak_aarch64
 	url = https://github.com/fujitsu/xbyak_aarch64