Fix Node deletion is too greedy

[Daniel53245]

Closes #2606

As mentioned in #2606 , in node editor mode, deleting nodes sometime results unwanted deletion of parent nodes.

This PR examines the algorithm for deletion and modified its behavior. Now, it will not cause the deletion in #2606, but I haven't tested it for other cases. I will put this as a draft PR till I am sure it works for other conditions

[Daniel53245]

Fix Node Deletion is too greedy

Original Code

https://github.com/GraphiteEditor/Graphite/blob/d39308c048cc439715943ff487294ffccaf12c70/editor/src/messages/portfolio/document/utility_types/network_interface.rs#L4177//

The original code for greedy node deletion was a graph search algorithm in rust. It iterates each upstream node for the nodes marked for deletion.

For one of the upstream node it checks all the child it connected to

1. the child is marked for deletion, search in this branch is stopped (will result deletion of upstream node if all the childs falls in this category)
2. the child is not marked for deletion, continue search the grandchildren

Cause of the original bug

As mentioned in the original issue, following images describes one of the condition that the algorithm fails and deleted "Identity" node while it is used by other nodes

During the deletion, the original algorithm check all the downstream node for the "Identity", it confirms that "equals is marked for deletion by the user. The algorithm searches through "Greater Than" and "Switch". A the point it reaches "Switch", the issue happened.

The line

let Some(downstream_nodes) = outward_wires.get(&current_node) else { continue };

"Switch" node does not have any child, this line continues the loop. The algorithm ignores "Switch" and mistake that all downstream nodes of "Identity" are marked for deletion

Modification

The original algorithm does a recursive search through the graph. I modified that algorithm such that it would mark the upstream node not for deletion once it reach a non-deletion downstream node. This is achieve by modifying line

if !delete_nodes.contains(downstream_id) {
	stack.push(downstream_node_output);
}

to

if !delete_nodes.contains(downstream_id) {
	can_delete = false;
	break;
}

Test of the modified code

I tested modified code in many situations

Deletion of the node will not cause deletion of any other node

Deletion of "Switch" cause both upstream node deleted

Deletion of "Logical Or" will not delete any other node

Sandwiched deletion will not cause the middle node to be deleted

Deleting last node of the chain will cause the deletion of all upstream nodes