Need a validation test for depth prepass.

It was recently discovered (https://github.com/BabylonJS/Babylon.js/pull/10466) that the [`needDepthPrePass`](https://doc.babylonjs.com/typedoc/classes/babylon.material#needdepthprepass) material property was causing the depth buffer to block the following opaque/color rendering step. We should consider adding a validation test to make sure this isn't broken in the future.

[The relevant code in renderingGroup.ts](https://github.com/BabylonJS/Babylon.js/blob/d120c8e483e3e92f01deba34880552be27bfaed8/src/Rendering/renderingGroup.ts#L353-L367)