Add RGB/CMY/Flat mode options for armor and skin layers

Replace normals scale setting and per-mode armor options with unified
RGB/CMY/Flat mode selection for armor, first skin layer, and second
skin layer. Flat mode renders solid gray with no lighting. Add toggle
to invert second skin layer colours.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
