This provides the following enhancements & bug fixes

This provides the following enhancements & bug fixes
- Fix the chequer-board bug.
- Add proper cloud coverage function - so scattered clouds are now truly scattered.
- Add real-time control for visibility range.
- Use a limited set of clouds rather than generating a completely new Geode for each cloud. This saves sorting and display time.
- Add controls to Rendering dialog to allow fine-tuning of the number of sprites, cloud visibility and the number of different types of cloud.
- Add some variance to the sort back-off to avoid all clouds being sorted at the same time.
- Pack attributes into vectors for performance
- Re-order the cloud type determination code so that if a cloud layer could either be stratus or cumulus, cumulus is used.
- Lowered the cloud level in the standard cloud configuration slightly so a cumulus layer is generated rather than stratus.

These last two mean that you should see some 3D cumuli if disabling real weather fetch.

My thanks to Yon Uriarte for his help with performance work.