This PR aims to solve all issues with `Groundcover` view distance handling in a satisfying way while preserving other optimisations that benefit other features. The main idea here is not to rely on `ViewData` updates for distance culling calculations because we can not accurately determine distance against lazily updated views. Instead, we perform an accurate measurement in a cull callback we can run every frame in `Groundcover` itself. While we do have to add some code to handle this feature, it is quite loosely coupled code that could be useful elsewhere in the future. These changes should address a performance regression @akortunov experienced.
I have been informed by @akortunov that my addition of `Groundcover::getViewDistance` is not working in some cases. Investigations revealed that some `ViewData` code interacting with my additions had been quite thoroughly optimised in a way that was not sufficiently documented and interfered with the new feature. With this PR we repair `getViewDistance` while preserving such optimisations and add a necessary comment to avoid issues in the future. In addition, we now rebuild views when the global `mViewDistance` changes.
- Cull only against bounding box, not bounding sphere, as this appears to perform better.
- Also traverse into non visible nodes to compute their LOD, this is to avoid nodes having to be loaded in when the player turns the view around, and will avoid unnecessary refs/unrefs of rendering data in the View. This should probably be turned off at some point for static cameras, such as the local maps.
