Metal setup merge (#1616)

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: Alex Cristici <alex.cristici@gmail.com>
Co-authored-by: mwilsnd <mwilsnd@gmail.com>
Co-authored-by: Stefan Karschti <stefan.karschti@gmail.com>
Co-authored-by: Bart Louwers <bart@emeel.net>
Co-authored-by: Alex Cristici <alex@skoon.energy>
Co-authored-by: Bart Louwers <bart.louwers@gmail.com>
Co-authored-by: Wei Chen <alanchenboy@gmail.com>
Co-authored-by: Siarhei Fedartsou <siarhei.fedartsou@gmail.com>
Co-authored-by: tdcosta100 <tdcosta100@gmail.com>

platform/default/include/mbgl/gfx/headless_backend.hpp

@@ -24,7 +24,11 @@ public:
     static std::unique_ptr<HeadlessBackend> Create(const Size size = {256, 256},
                                                    SwapBehaviour swapBehavior = SwapBehaviour::NoFlush,
                                                    const gfx::ContextMode contextMode = gfx::ContextMode::Unique) {
+#if MLN_RENDER_BACKEND_METAL
+        return nullptr;
+#else // MLN_RENDER_BACKEND_OPENGL
         return Backend::Create<HeadlessBackend, Size, SwapBehaviour, gfx::ContextMode>(size, swapBehavior, contextMode);
+#endif
     }
 
     virtual PremultipliedImage readStillImage() = 0;