Merge #1591

1591: Expose calayer r=kvark a=Sineaggi

**Description**

This pr restores the ability to create a surface from a pre-existing CAMetalLayer. This is required for wgpu-native.

Co-authored-by: Clayton Walker <cwalker@sofi.org>

wgpu-core/src/instance.rs

@@ -376,7 +376,6 @@ impl<G: GlobalIdentityHandlerFactory> Global<G> {
         id.0
     }
 
-    /*TODO: raw CALayer handling
     #[cfg(metal)]
     pub fn instance_create_surface_metal(
         &self,
@@ -396,7 +395,7 @@ impl<G: GlobalIdentityHandlerFactory> Global<G> {
         let mut token = Token::root();
         let id = self.surfaces.prepare(id_in).assign(surface, &mut token);
         id.0
-    }*/
+    }
 
     pub fn surface_drop(&self, id: SurfaceId) {
         profiling::scope!("drop", "Surface");

wgpu-hal/src/metal/mod.rs

@@ -63,6 +63,17 @@ impl crate::Api for Api {
 
 pub struct Instance {}
 
+impl Instance {
+    pub fn create_surface_from_layer(
+        &self,
+        layer: &mtl::MetalLayerRef,
+    ) -> Surface {
+        unsafe {
+            Surface::from_layer(layer)
+        }
+    }
+}
+
 impl crate::Instance<Api> for Instance {
     unsafe fn init(_desc: &crate::InstanceDescriptor) -> Result<Self, crate::InstanceError> {
         //TODO: enable `METAL_DEVICE_WRAPPER_TYPE` environment based on the flags?