Add layers, delta undo, shape symmetry, and reference overlay to texture editor

[CoderJoeW]

Summary

• Layer system: each face now supports multiple layers with per-layer opacity and visibility. New layer panel with add, delete, merge down, and reorder controls. Compositing flattens visible layers for display/save with a fast path for the single-layer case.
• Delta-based undo/redo: tracks dirty regions during painting and stores only changed pixel crops instead of full image snapshots. Undo limit raised from 20 to 100.
• Shape tool symmetry: line, rectangle, and ellipse tools now respect horizontal, vertical, and quad mirror modes via mirrored start/end point pairs.
• Gradient selection constraint: gradient tool fills only within the active selection rectangle when one exists, enabling localized gradient fills.
• Reference image overlay: load any PNG/JPG/BMP as a semi-transparent tracing guide on the canvas with adjustable opacity. Controls are hidden until a reference is loaded.