improves groundcover view distance

[bosvensson1]

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.

[bosvensson1]

This PR currently experiences a bug associated with vertexLodMod because vertexLodMod code has been duplicated and misplaced and we have multiple uses of lodLevel with different semantics. I will need to refactor lodLevel as well before this PR can work properly.

[bosvensson1]

This PR should be complete now, but it could warrant some testing against different Groundcover view distances, vertex lod mod and lod factor values.

[akortunov]

While this PR seems to do its main purpose, I noticed terrain seams which do not present in the same place in master branch:

[Terrain]
distant terrain = true

It is the hill directly to north from Fort Pelagiad. Also overall terrain details seem to be worse with this PR.

[AnyOldName3]

The general idea here looks good to me. Obviously, we want to figure out the cause of the terrain degradation, though.

[bosvensson1]

Thank you for the detailed testing. We appear to have experienced this degradation because my changes have revived some dead code in another module, but instead of being gifted additional functionality we have been gifted additional bugs in this case. I have now disabled this broken code and included some further cleanup. These latest changes should now work properly. It would be nice to test precisely the location on the above screenshot again if we have managed to save its coordinates.

[psi29a]

Smoke testing and review of the effected area shows that seems issues have been resolved with a nice boost in performance. Thanks @bosvensson1 and @akortunov

[bosvensson1]

We appear to have slipped in a mistake at the last minute that was not noticed because I had just tested (0, 0) origin coordinates and @psi29a did not use a Groundcover setup. For those experiencing issues please try to delete the following line from groundcover.cpp:
box = osg::BoundingBox(box._min+worldCenter, box._max+worldCenter);

[psi29a]

That was my fault, I was trying to match up to the screenshoot from @akortunov ; I have validated again with the change.
This time with groundcover...

I will push the fix.