improves groundcover view distance (#3219)

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.

components/terrain/viewdata.cpp

@@ -145,7 +145,6 @@ ViewData *ViewDataMap::getViewData(osg::Object *viewer, const osg::Vec3f& viewPo
             }
             vd->setViewPoint(viewPoint);
             vd->setActiveGrid(activeGrid);
-            vd->setChanged(true);
             needsUpdate = true;
         }
     }