Metal: implement texture_create_from_extension

Parameters p_type, p_format, p_array_layers, p_depth_stencil are
ignored - MTLTexture (and the callee) already have this information
and is only relevant when reinterpreting or remaping the texture in
different ways.

drivers/metal/rendering_device_driver_metal.mm

@@ -358,7 +358,11 @@ RDD::TextureID RenderingDeviceDriverMetal::texture_create(const TextureFormat &p
 }
 
 RDD::TextureID RenderingDeviceDriverMetal::texture_create_from_extension(uint64_t p_native_texture, TextureType p_type, DataFormat p_format, uint32_t p_array_layers, bool p_depth_stencil) {
-	ERR_FAIL_V_MSG(RDD::TextureID(), "not implemented");
+	id<MTLTexture> obj = (__bridge id<MTLTexture>)(void *)(uintptr_t)p_native_texture;
+
+	// We only need to create a RDD::TextureID for an existing, natively-provided texture.
+
+	return rid::make(obj);
 }
 
 RDD::TextureID RenderingDeviceDriverMetal::texture_create_shared(TextureID p_original_texture, const TextureView &p_view) {