CSG flicker rendering bug with all union, one subtraction box

[golddotasksquestions]

Godot version:
Godot 3.2 alpha3

OS/device including version:
Win7 Nvidia

Issue description:
Stable visual positional flicker on the mesh with almost overlapping CSG boxes with one subtraction box covering the area (outerbox)

Steps to reproduce:
Move the CSG boxes on the z-axis until they almost overlapp in the minimal project:

Minimal reproduction project:
csg_flicker_bug.zip

[golddotasksquestions]

Though it does not look quite as bad anymore, this issue is still present in Godot 3.2.3 stable and Godot 3.2.4 beta2:
@akien-mga please reopen.

Steps to replicate:

1. Create a new 3D scene,
2. Add CSGCombiner node as child of the Spacial,
3. Add CSGBox as child of the Combiner (floor), resize,
4. Add another CSGBox as child of the Combiner (wall), resize and move,
5. duplicate and move the wall a few times.

Issue is also present if all CSGBoxes have default Inspector values (Union).

There is also a very noticeable reduction in Editor framerate with only 10 CSG boxes in the scene.

[Calinou]

@golddotasksquestions Please open a new issue with a minimal reproduction project attached.