Expose MeshPortalMaterial RenderTarget

[frankievx]

Describe the feature you'd like:

I'm working with the MeshPortalMaterial and I'm interested in running a custom shader effect that is dependent on an initial snapshot of the scene in order to define a color palette. In order to accomplish this, I need access to the internal portal's render target in order to read the pixels. I think the benefit of exposing the render target allows for more control and the ability to apply postprocessing effects to the portal.

Suggested implementation:

I'd be happy to implement this however would want some feedback on if this is a good idea or not before i spend the time and effort.