get the metal backend to compile (does not load)

projects/apis/metal/loader/metal_instance_loader.c

@@ -0,0 +1,15 @@
+#include "metal_loader.h"
+
+gnInstanceFunctions loadMetalInstanceFunctions() {
+    return (gnInstanceFunctions){
+        ._gnCreateInstance = NULL,
+        ._gnDestroyInstance = NULL,
+        ._gnGetPhysicalDevices = NULL,
+        ._gnQueueCanPresentToSurface = NULL,
+        ._gnCreateOutputDevice = NULL,
+        ._gnDestroyOutputDevice = NULL,
+        ._gnCreateMacOSWindowSurface = NULL,
+        ._gnDestroyWindowSurface = NULL,
+        ._gnGetSurfaceDetails = NULL
+    };
+}