normal-map[css]: Add flip-y checkbox for normals.
normal-map[css]: Add multi-image loading on drop.

Simple heuristic (‘normal’ in file name) to determine which file is
diffuse/normal.
normal-map[css]: Update canvas previews on config input.

Copy over light configuration values from:
https://github.com/mattdesl/lwjgl-basics/wiki/ShaderLesson6