fixes getViewDistance

[bosvensson1]

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.

[akortunov]

This PR seems to degrade performance a lot when groundcover is used. I have a scene where I get an FPS drop from 75 to 35 due to twice larger GPU usage.

https://imgsli.com/Nzk3MjQ

An issue ticket on our bugtracker: #6377

[psi29a]

This is an incredible hit to performance, it needs to be fixed rather sooner than later. Otherwise I'm going to have to revert his @bosvensson1 . :(

Any ideas?

[bosvensson1]

I will have to rethink getViewDistance approach here and it will take some time in light of other work that has accumulated. If we prefer pop in issues over such a performance hit I suppose we can revert this PR in the meantime.

[akortunov]

Personally I perefer 0.47 behaviour (no performance hit and no pop-ins). A behaviour prior to #3126 seemed to work as well.

[bosvensson1]

Unquestionably, yes. Regretfully the 0.47 behaviour negatively impacted the behaviour of several non groundcover features.