viewport BFRES mouse selection

- [ ] BFRES shader for drawing color ID pass.
- [ ] Draw BFRES polygon index to a color ID pass (see `NUD.cs` for an example).
- [ ] Convert the color at mouse position to the appropriate BFRES polygon in the mesh list.

Start by inspecting NUD.cs for the color-ID rendering example, then trace the BFRES viewport rendering and mesh-list entry points. Done means BFRES polygons render distinct color IDs, and clicking a viewport pixel resolves that color to the appropriate polygon in the mesh list.

Written by the indexing model from the issue text.