gl_showtris depth mode

[Paril]

Most Q1 engines have some method of using gl_showtris while also respecting the depth buffer; I modified my local q2pro to do this since it makes inspecting BSP face output a lot easier, although the way I did it is kinda flickery. Would be a nice simple addition.

[Calinou]

although the way I did it is kinda flickery

Can you use glPolygonOffset() avoid this issue?

I've been meaning to implement something similar in Godot but could never get it to work though, especially in Vulkan...