Allow creating a surface by passing a metal layer.

This is useful if you're using `SDL2`
```rust
    let canvas = window.into_canvas().build().unwrap();
    let metal_layer = unsafe { sdl2::sys::SDL_RenderGetMetalLayer(canvas.raw()) }; 
    let surface = instance.create_surface_with_metal_layer(metal_layer);
```

wgpu-rs/src/lib.rs

@@ -242,6 +242,13 @@ impl Instance {
             id: wgn::wgpu_instance_create_surface_from_winit(self.id, window),
         }
     }
+    
+    #[cfg(feature = "metal")]
+    pub fn create_surface_with_metal_layer(&self, window: *mut std::ffi::c_void) -> Surface {
+        Surface {
+            id: wgn::wgpu_instance_create_surface_from_macos_layer(self.id, window),
+        }
+    }
 }
 
 impl Adapter {