New Tree relationship lines disappear when the selected node is significantly out of view

[Calinou]

Godot version: master Git b6f3814

OS/device including version: Fedora 33, GeForce GTX 1080 (NVIDIA 465.27)

Issue description:

The new Tree relationship lines disappear when the selected node is significantly out of view.

cc @pycbouh

Before scrolling up

After scrolling up

Steps to reproduce:

• Create a scene with dozens of nodes.
• Select a node at the bottom.
• Scroll up.

Minimal reproduction project: N/A

[YuriSizov]

Well that's unfortunate. This is because the whole line is drawn from the selected item, and I didn't realize we have this kind of optimization to not draw it when out of view. Though maybe not a big deal, as I already do the necessary checks to draw each segment, I think.