macOS: Implement fallback from Metal to Vulkan for x86_64

Author: SheepYhangCN

doc/classes/ProjectSettings.xml

@@ -2982,8 +2982,8 @@
 			[b]Note:[/b] This setting is implemented only on Windows, Android, macOS, iOS, and Linux/X11.
 		</member>
 		<member name="rendering/rendering_device/fallback_to_vulkan" type="bool" setter="" getter="" default="true">
-			If [code]true[/code], the forward renderer will fall back to Vulkan if Direct3D 12 is not supported.
-			[b]Note:[/b] This setting is implemented only on Windows.
+			If [code]true[/code], the forward renderer will fall back to Vulkan if Direct3D 12 (on Windows) or Metal (on macOS x86_64) are not supported.
+			[b]Note:[/b] This setting is implemented only on Windows and macOS.
 		</member>
 		<member name="rendering/rendering_device/pipeline_cache/enable" type="bool" setter="" getter="" default="true">
 			Enable the pipeline cache that is saved to disk if the graphics API supports it.

platform/macos/display_server_macos.mm

@@ -3785,6 +3785,14 @@
 
 #if defined(RD_ENABLED)
 #if defined(VULKAN_ENABLED)
+#if defined(__x86_64__)
+	bool fallback_to_vulkan = GLOBAL_GET("rendering/rendering_device/fallback_to_vulkan");
+	// Metal rendering driver not available on Intel.
+	if (fallback_to_vulkan && rendering_driver == "metal") {
+		rendering_driver = "vulkan";
+		OS::get_singleton()->set_current_rendering_driver_name(rendering_driver);
+	}
+#endif
 	if (rendering_driver == "vulkan") {
 		rendering_context = memnew(RenderingContextDriverVulkanMacOS);
 	}