Fix NOTIFICATION_POSTINITIALIZE sent twice to native parent classes

[dsnopek]

As brought up on PR godotengine/godot#91018, we're currently sending NOTIFICATION_POSTINITIALIZE to the native parent class twice.

This issue was introduced by PR #1269, which was attempting to get the NOTIFICATION_POSTINITIALIZE to arrive to the extension class by sending it in Wrapped::_postinitialize(), but the way it's doing it will send the notification through the native parent class again.

This PR makes it so that Wrapped::_postinitialize() will only send NOTIFICATION_POSTINITIALIZE through the class hierarchy that exists purely on the GDExtension side. Because NOTIFICATION_POSTINITIALIZE is run on the parent class first, this effectively just picks up where the previous notification that went through the native parent class heirarchy stopped.

PR godotengine/godot#91018 and godotengine/godot#91019 attempt to more comprehensively fix NOTIFICATION_POSTINITIALIZE, but those require changing the GDExtension interface (which is Godot 4.4 material at this point), and we need a fix to this issue that can be used with Godot 4.1, 4.2 and 4.3.

[AThousandShips]

We should be able to test for this with a flag and a notification check, just to validate it

[dsnopek]

We should be able to test for this with a flag and a notification check, just to validate it

There are already automated tests for ensuring that NOTIFICATION_POSTINITIALIZE goes to the extension class - they were added in PR #1269 and they still pass with this PR :-)

I don't think there's a way for us to test that the bug this PR is fixing is fixed, though, since that all happens on the Godot side. So, I don't think this PR needs to add any more tests.

[AThousandShips]

Meant like adding something in the existing check so:

	if (p_what == NOTIFICATION_POSTINITIALIZE) {
		if (post_initialized) {
			// Some error.
		}
		post_initialized = true;
	}

Since the issue was sending it twice yes?

Unless it's only sent to the parent twice

[dsnopek]

Unless it's only sent to the parent twice

It's only sent to the native parent class twice. On the godot-cpp side, we only see it once.

[Naros]

Nice catch @dsnopek !

[vnen]

Approved in the GDExtension meeting.

[dsnopek]

Cherry-picked for 4.2 in PR #1465

[dsnopek]

Cherry-picked for 4.1 in PR #1466