[3.x] Fix CanvasItem visibility propagation

[timothyqiu]

Fixes #58251

I believe the logic is also wrong on master, see #58251 (comment). But I'm not sure if a similar fix would cancel the optimization of #57684 on master. Since the issue is currently only reproducible on 3.x, this PR only targets 3.x.

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The name of p_was_visible parameter is no accurate. It's only "was visible" if the node is the source of the propagation. It's always false for other nodes in the propagation chain, making the name awkward. The only place that passes the parameter is set_visible():

godot/scene/2d/canvas_item.cpp

Line 393 in 7384475

_propagate_visibility_changed(p_visible, !p_visible);

When used, it only makes sense if it's true:

godot/scene/2d/canvas_item.cpp

Lines 365 to 367 in 7384475

	} else if (!p_visible && (visible || p_was_visible)) {
	emit_signal(SceneStringNames::get_singleton()->hide);
	}

So I changed it to p_is_source so it's easier to understand. The above code will emit the signal when it's propagating "hide" and if the current node is visible or the current node is the source of propagation.

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The actual cause of the issue is line 360:

godot/scene/2d/canvas_item.cpp

Lines 356 to 361 in 7384475

	void CanvasItem::_propagate_visibility_changed(bool p_visible, bool p_was_visible) {
	if (p_visible && first_draw) { //avoid propagating it twice
	first_draw = false;
	}
	parent_visible_in_tree = p_visible;
	notification(NOTIFICATION_VISIBILITY_CHANGED);

1. When toggling visibility of a CanvasItem, this syncs parent_is_visible_in_tree with it. This should only happen for other nodes in the propagation chain, not for the node itself.
2. For other nodes in the propagation chain, p_visible is the visible of the propagation source, it does not have to be the same as its is_visible_in_tree(). So a proper check like in NOTIFICATION_ENTER_TREE should be done.

[akien-mga]

Superseded by #58386.