macOS: ARC bridge casts for FlutterMetalTexture.user_data

[cbracken]

In [FlutterSurface asFlutterMetalTexture] we return a FlutterMetalTexture whose user_data field holds a void* reference to the associated FlutterSurface.

For conistency with other parts of the codebase, this now uses ARC bridge casts to perform the additional retain and release of the associated surface rather than CoreFoundation functions CFBridgingRetain and CFBridigingRelease. In FlutterEngine, for example:

engine/shell/platform/darwin/macos/framework/Source/FlutterEngine.mm

Lines 648 to 664 in a672f97

	// Retain for use in post_task_callback. Released in destruction_callback.
	.user_data = (__bridge_retained void*)self,
	.runs_task_on_current_thread_callback = [](void* user_data) -> bool {
	return [[NSThread currentThread] isMainThread];
	},
	.post_task_callback = [](FlutterTask task, uint64_t target_time_nanos,
	void* user_data) -> void {
	FlutterEngine* engine = (__bridge FlutterEngine*)user_data;
	[engine postMainThreadTask:task targetTimeInNanoseconds:target_time_nanos];
	},
	.identifier = ++sTaskRunnerIdentifiers,
	.destruction_callback =
	[](void* user_data) {
	// Balancing release for the retain when setting user_data above.
	FlutterEngine* engine = (__bridge_transfer FlutterEngine*)user_data;
	engine = nil;
	},

Also migrates the code to initialise and immediately return the struct using field designators, and reorders the assignments to be in the declaration order specified in embedder.h as required by the C++ spec.

No changes to tests since this introduces no semantic change.

Issue: flutter/flutter#137801

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[flutter-dashboard]

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[stuartmorgan-g]

test-exempt: code refactor with no semantic change

[stuartmorgan-g]

LGTM