Optimize grass rendering with GPU scatter and indirect draws

[vanlueckn]

Summary

• Replace CPU-generated grass meshes with GPU scatter compute and indirect draws.
• Add a cached ground height/weight texture to place grass and exclude fields, roads, and water.
• Add configurable grass range and density for Low, Medium, and High quality settings.
• Preserve weather, lighting, wind animation, seasonal behavior, and terrain-based grass variation.
• Document the new renderer and update grass-quality localization text.

Testing

• Not run; no test results were provided.