[Request] Custom Normal Map input And Submaterial support (Features from Advanced Material 2)

I dont know how hard this is to add with the rewritten material creation stuff, but Advanced material 2 had the ability for you to input your own normal map texture and also sub material selection (slider at the top of the tool menu to select which sub material id to place the texture on)

would be great if these were added especialyl for custom normal-maps, as some textures dont actually have one/have a normal map that looks ugly

Start by locating the rewritten material creation flow and the tool menu controls, then compare their available inputs with the Advanced Material 2 behavior described here. Done would mean supporting custom normal-map textures and selecting the submaterial ID to receive the texture, with the scope and validation approach clarified first.

Written by the indexing model from the issue text.