use stackgl modules

Would be good to replace ThreeJS with some modules to remove some of the "magic" (e.g. the shader has some uniforms/attributes defined deep in ThreeJS, weird stuff like `USE_MAP` is needed, etc).

Start by reviewing the repository's current ThreeJS integration and how shader uniforms and attributes are supplied. The issue does not name files or tests; completion would require replacing the ThreeJS dependency with StackGL modules while preserving the normal-map perturbation behavior and removing the hidden configuration it describes.

Written by the indexing model from the issue text.