Auto clean up geometry after delete RoadPoint

[TheDuckCow]

Right now, if a child RoadPoint is deleted, geometry gets left behind until the whole road network (and also some odd unexpected geoemtry is added).

Ideally we can detect a delete signal, and automatically clean up any references to points. This may be tightly coupled to: https://github.com/TheDuckCow/godot-road-generator/issues/64

[bdog2112]

This problem was bad in the past. But, it seemed much improved, lately. My first attempt at recreating the problem failed. (Segments updated just fine.) So, I brainstormed and came up with some steps that exhibited problems. Please let me know if this is not the problem you're describing.

Steps to recreate problem:

• Create new scene with RoadNetwork as root note
• Create 2x2 road
• Extend road so that there are 5 RoadPoints
• Delete RP_003 in the Scene dock
• Outcome: Segment is removed in the 3D Viewport, as expected. But, numerous errors are displayed in the console. Additionally, "Undo" brings back the RoadPoint in the Scene dock. But, it is not redrawn in the 3D Viewport and more errors are displayed.

[bdog2112]

Presumably, the initial errors have something to do with Segments trying to redraw deleted RoadPoints. Then, Undo/Redo also has a problem dealing with deleted RoadPoints.

In order for Undo/Redo to work, we may need to keep "deleted" objects in memory rather than actually deleting them. They could be purged at some arbitrary later time such as on save.

Also, have to look and see what kind of control Godot gives us to manage/prevent the actual deletion of objects.

[TheDuckCow]

Regarding "need to keep "deleted" objects in memory", there are actually handles for this in the undo/redo class, but I was not successful in getting it to work fully or at least was a challenge to wrangle (add_do_reference and add_undo_reference). For simplicity, we can leave out undo/redo considerations for this task.

I do also agree that I recently improved this quite a bit with this commit https://github.com/TheDuckCow/godot-road-generator/pull/83/commits/949159444d2bd34c2c456916789955b02102b973 (merged), this issue was created before that time but indeed there's still a ton of errors that print out right now. Ideally we can find a way to eliminate those. I'm fine for that to be the scope of this task if you're aligned, I'll rename it accordingly.

... oddly for me, when I undo after a delete, the node comes back into the dock and it somes redraws the geometry in the scene (though with lots of reference errors). First time it did, second time not. Happy to defer that to another out of scope task.