Allow SurfaceTool to add data to INSTANCE_CUSTOM for shader

[orosmatthew]

I am trying to write a shader and need to send just 3 more values to the shader for each vertex (which would massively simplify the whole process). The docs say that this value is mostly used for particles but since I am not using particles, it would be great if I could use those values instead for a different purpose. It would be optimal if I could set these values using the SurfaceTool

[clayjohn]

I am assuming you figured out the correct answer yourself. But as a note for others who may struggle with the same problem. INSTANCE_CUSTOM is only used by Particles and MultiMeshes. To set it yourself, you can use the MultiMesh function set_instance_custom_data. The reason it can't be accessed through the surfacetool is that the surfacetool sets per-vertex parameters on the surface, while INSTANCE_CUSTOM is a per-instance variable that can only be set on MultiMeshes.

Hope that helps :)