Expose macOS -[FlutterViewController initWithEngine:::] in header

[jmagman]

To support macOS add-to-app, move initWithEngine: into header so the host app can run the warm up the engine and pass it into a view controller later to display the Flutter views.

This will match the iOS pattern documented in https://docs.flutter.dev/development/add-to-app/ios/add-flutter-screen?tab=engine-swift-tab#show-a-flutterviewcontroller-with-your-flutterengine

Fixes https://github.com/flutter/flutter/issues/105347

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[stuartmorgan]

I haven't had a chance to look yet (I'm OOO today) but I'm pretty sure anyone trying to do what the PR describes enabling will hit https://github.com/flutter/flutter/issues/74735

So we should probably fix that first.

[awood314]

I noticed that initWithEngine calls setViewController on the engine, but setViewController will get called again by launchEngine

This causes the view not to show, because the viewController has not allocated its flutterView yet, so that's what is passed to the renderer and is never reassigned. I think the proper fix would be to remove the setViewController call from initWithEngine?

[cbracken]

Sorry; I never linked back here when I commented on the other issue. See notes here: https://github.com/flutter/flutter/issues/74735#issuecomment-1157931202

[chinmaygarde]

This looks blocked. Is that right @jmagman & @cbracken ?

[cbracken]

We'll want to fix https://github.com/flutter/flutter/issues/74735 first, yes. Added some notes on that bug the other day. Marked as WIP to get it off your PR triage list till then.

[Hixie]

@jmagman What's the status of this PR?

[jmagman]

Closing in favor of https://github.com/flutter/engine/pull/36410