Add FlutterMetalLayer as optional alternative to CAMetalLayer (#48226)

This PR implements `FlutterMetalLayer`, a drop-in (as far as Flutter is
concerned) replacement for `CAMetalLayer`. The biggest difference is
that `FlutterMetalLayer` can present frames from background thread
within a `CATransaction`.

`FlutterMetalLayer` is disabled by default. To opt-in, add the following
item to `Info.plist`:

```xml
        <key>FLTUseFlutterMetalLayer</key>
	<true/>
```

The performance seems quite good, consistent 120hz on iPhone 13 Pro.

Benefits
- presenting with transaction from background thread, which, down the
line, would allow for platform views without thread merging.
- fine control over how the surface is displayed - we can display single
surface on multiple `CALayers`, each showing different part, allowing
for performant implementation of unobstructed platform views.
 
Drawbacks
- this not being a metal layer makes working with metal instrument tools
more awkward

Author: knopp

ci/licenses_golden/licenses_flutter

@@ -6626,6 +6626,9 @@ ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterKeySe
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterKeyboardManager.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterKeyboardManager.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterKeyboardManagerTest.mm + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterMetalLayer.h + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterMetalLayer.mm + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterMetalLayerTest.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterOverlayView.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterOverlayView.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterPlatformPlugin.h + ../../../flutter/LICENSE
@@ -9452,6 +9455,9 @@ FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterKeySeco
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterKeyboardManager.h
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterKeyboardManager.mm
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterKeyboardManagerTest.mm
+FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterMetalLayer.h
+FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterMetalLayer.mm
+FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterMetalLayerTest.mm
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterOverlayView.h
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterOverlayView.mm
 FILE: ../../../flutter/shell/platform/darwin/ios/framework/Source/FlutterPlatformPlugin.h

impeller/renderer/backend/metal/surface_mtl.mm

@@ -264,7 +264,12 @@
       [command_buffer waitUntilScheduled];
       [drawable_ present];
     } else {
-      [command_buffer presentDrawable:drawable_];
+      // The drawable may come from a FlutterMetalLayer, so it can't be
+      // presented through the command buffer.
+      id<CAMetalDrawable> drawable = drawable_;
+      [command_buffer addScheduledHandler:^(id<MTLCommandBuffer> buffer) {
+        [drawable present];
+      }];
       [command_buffer commit];
     }
   }

shell/platform/darwin/ios/BUILD.gn

@@ -58,12 +58,17 @@ source_set("flutter_framework_source_arc") {
   public_configs = [ "//flutter:config" ]
 
   sources = [
+    "framework/Source/FlutterMetalLayer.h",
+    "framework/Source/FlutterMetalLayer.mm",
     "framework/Source/FlutterTextInputDelegate.h",
     "framework/Source/FlutterTextInputPlugin.h",
     "framework/Source/FlutterTextInputPlugin.mm",
   ]
 
-  frameworks = [ "UIKit.framework" ]
+  frameworks = [
+    "UIKit.framework",
+    "IOSurface.framework",
+  ]
 }
 
 source_set("flutter_framework_source") {
@@ -257,6 +262,7 @@ shared_library("ios_test_flutter") {
     "framework/Source/FlutterFakeKeyEvents.h",
     "framework/Source/FlutterFakeKeyEvents.mm",
     "framework/Source/FlutterKeyboardManagerTest.mm",
+    "framework/Source/FlutterMetalLayerTest.mm",
     "framework/Source/FlutterPlatformPluginTest.mm",
     "framework/Source/FlutterPlatformViewsTest.mm",
     "framework/Source/FlutterPluginAppLifeCycleDelegateTest.mm",

shell/platform/darwin/ios/framework/Source/FlutterMetalLayer.h

@@ -0,0 +1,26 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#import <QuartzCore/QuartzCore.h>
+
+/// Drop-in replacement (as far as Flutter is concerned) for CAMetalLayer
+/// that can present with transaction from a background thread.
+@interface FlutterMetalLayer : CALayer
+
+@property(nullable, retain) id<MTLDevice> device;
+@property(nullable, readonly) id<MTLDevice> preferredDevice;
+@property MTLPixelFormat pixelFormat;
+@property BOOL framebufferOnly;
+@property CGSize drawableSize;
+@property BOOL presentsWithTransaction;
+@property(nullable) CGColorSpaceRef colorspace;
+@property BOOL wantsExtendedDynamicRangeContent;
+
+- (nullable id<CAMetalDrawable>)nextDrawable;
+
+/// Returns whether the Metal layer is enabled.
+/// This is controlled by FLTUseFlutterMetalLayer value in Info.plist.
++ (BOOL)enabled;
+
+@end