Split up MainPass3dNode

[JMS55]

What problem does this solve or what need does it fill?

Upcoming render features want to optionally insert rendering logic in between the opaque/alpha/transparent phases.

What solution would you like?

Split MainPass3dNode into two separate nodes: opaque + alpha, and transparent.

Alternatives

Add user defined callbacks that run in between the passes, to keep everything in the same node, and reuse render passes.

Additional context

• First-party skybox support #8008
• StandardMaterial Light Transmission #8015

[superdump]

Yup. Though importantly they need to reuse render passes where possible. Opaque and alpha mask should reuse the same render pass for example.

[JMS55]

We don't currently do that, but as part of splitting the nodes up we could do so.