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>

src/mbgl/programs/program.hpp

@@ -14,7 +14,7 @@
 #include <unordered_map>
 
 #include <mbgl/shaders/shader_manifest.hpp>
-#ifdef MBGL_RENDER_BACKEND_OPENGL
+#if !MLN_RENDER_BACKEND_METAL
 #include <mbgl/gl/program.hpp>
 #endif
 
@@ -54,7 +54,11 @@ public:
 
     Program(const ProgramParameters& programParameters) {
         switch (gfx::Backend::GetType()) {
-#ifdef MBGL_RENDER_BACKEND_OPENGL
+#if MLN_RENDER_BACKEND_METAL
+            case gfx::Backend::Type::Metal: {
+                break;
+            }
+#else // MLN_RENDER_BACKEND_OPENGL
             case gfx::Backend::Type::OpenGL: {
                 program = std::make_unique<gl::Program<Name>>(programParameters.withDefaultSource(
                     {gfx::Backend::Type::OpenGL,