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/chunkmanager.cpp

@@ -52,6 +52,9 @@ struct FindChunkTemplate
 
 osg::ref_ptr<osg::Node> ChunkManager::getChunk(float size, const osg::Vec2f& center, unsigned char lod, unsigned int lodFlags, bool activeGrid, const osg::Vec3f& viewPoint, bool compile)
 {
+    // Override lod with the vertexLodMod adjusted value.
+    // TODO: maybe we can refactor this code by moving all vertexLodMod code into this class.
+    lod = static_cast<unsigned char>(lodFlags >> (4*4));
     ChunkId id = std::make_tuple(center, lod, lodFlags);
     osg::ref_ptr<osg::Object> obj = mCache->getRefFromObjectCache(id);
     if (obj)