TerrainRGB, Terrarium Plugins: Further Improvements

Related to #1690

Issues

• The current implementation does not handle raycasting / levels so well or flexibly, resulting in skipped LoDs. Instead of trying to expand children into the hierarchy it would be best to create multiple sub-meshes per tile and run per-mesh bounding box checks to accelerate raycasting
• Adjusting the height scale means every bounding box must be expanded. It would be best to do this as-needed or as-traversed, instead, with a kind of dirty check.
• The height altitude should be in a consistent unit and / or scale relative to the scale of the stretched scale of the world around it (either consistent based on camera position or scaled based on locality)

Functionality / Perf

• Core bounding-volume update infrastructure in the tiles renderer (grow-only bounds, animation, parent-child consistency, as-traversed dirty updates instead of full traversal on heightScale change)
• Debug bound visualizations refreshing after heightScale changes (falls out of the above)
• Improve accounting of downloaded DEM memory (currently double counted on sub meshes with shared items)
• Avoid regenerating a mesh on raycast
• Only update bounds when traversed or enable dynamic bound checking to avoid perf hit when updating height scale

Effects

• Hillshade-style shading effect
• Concavity map revisit (dynamically generated from DEM)

Other

• SkirtedPlaneGeometry adoption in GeneratedSurfacePlugin
• Increase skirt length?
• Upstreaming the bump map chunk fixes to three.js
• Frustum culling support (likely unnecessary unless users request it since it's disabled by default - requires updating bounds)

Start by reading the TerrainRGB and Terrarium plugin implementation related to #1690, then trace the tiles renderer's raycasting, bounds-update, DEM memory, and heightScale paths. Review GeneratedSurfacePlugin and SkirtedPlaneGeometry as additional entry points. The issue needs one scoped improvement and explicit acceptance criteria before completion can be determined.