Terrain: Consider rendering terrain occlusion depth to 1/4 screen resolution size

Nobody has claimed this yet.

As globe gets introduced, we add more rendering workload. A potential terrain optimization is to render the terrain depth pass used for symbol occlusion at a lower resolution (1/4). Symbol occlusion doesn't need the full precision of the screen resolution and occlusion results are more forgiving of false positive results.
- Evaluate current GPU terrain depth pass cost
- Evaluate whether terrain depth could be rendered at a smaller viewport resolution without introducing rendering issues
- Evaluate whether the method should be adopted by globe

Start by locating the terrain depth pass and measuring its current GPU cost. Evaluate rendering that pass at one-quarter screen resolution, checking for symbol-occlusion issues and whether the approach also works with globe rendering. Done means the performance impact and rendering trade-offs are documented, with a clear adoption decision.

Written by the indexing model from the issue text.