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/buffercache.cpp

@@ -12,8 +12,8 @@ namespace
 template <typename IndexArrayType>
 osg::ref_ptr<IndexArrayType> createIndexBuffer(unsigned int flags, unsigned int verts)
 {
-    // LOD level n means every 2^n-th vertex is kept
-    size_t lodLevel = (flags >> (4*4));
+    // LOD level n means every 2^n-th vertex is kept, but we currently handle LOD elsewhere.
+    size_t lodLevel = 0;//(flags >> (4*4));
 
     size_t lodDeltas[4];
     for (int i=0; i<4; ++i)