CSS2DRenderer: Add sortObjects.

[Konseyy]

Fixed #31945.

Description

Implemented a way of opting out from CSS2DRenderer automatic z-index assignment

[Konseyy]

Revised to have depthTest property on CSS2DObject. This allows all objects to still be assigned z-index values, but allows certain objects to opt out of distance-to-camera calculations.
Objects with depthTest = false will get assigned z-index values based on their renderOrder, but multiple objects with the same renderOrder and depthTest = false will have the same z-index.

Current approach puts objects with depthTest = false in front of objects with depthTest = true to mimic normal rendering

[Mugen87]

I think these changes make the renderer unnecessarily complex. If you need such a fine-granular control, I suggest your fork the renderer and use a custom version. IMO, the only change that would be consistent is introducing renderer.sortObjects.

[Konseyy]

Understandable, changed the implementation to just sortObjects

[Mugen87]

@yomotsu Are you okay with this change?

The idea is to add sortObjects to CSS2DRenderer, a flag that is known from WebGLRenderer. In context of CSS2DRenderer, setting it to false gives the developer full control over z-index.