Z-fighting occurs with collision shape fill or when a CSG node is selected due to gizmo drawing

[fire]

Tested versions

• Reproducible in Godot Engine 4.3 stable
• Reproducible in Godot Engine master 76fa7b2

System information

Windows 11

Issue description

#94321 @aaronfranke mentioned that CSG has Z-fighting when a CSG node is selected

Steps to reproduce

Select the second csg node and fly around.

Recording.2024-11-13.091923.mp4

Minimal reproduction project (MRP)

csg_flicker.zip

[JekSun97]

I also mentioned this problem here #96969

[Calinou]

This can be fixed by offsetting the gizmo's vertices in a way similar to the BaseMaterial3D's grow property. This is not currently exposed in the 3D material gizmos, so it should be done (or implemented manually on the CPU, but that would be slower).

If only we had glPolygonOffset() in modern low-level graphics APIs... It would greatly simplify the implementation of godotengine/godot-proposals#1000 as well.