Merge fix for CTerrainSceneNode::preRenderLODCalculations() to trunk; take the camera's up vector into account when determining whether terrain scene node visibility needs to be rechecked.  New test added; thanks to Travis for another great test case.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@2003 dfc29bdd-3216-0410-991c-e03cc46cb475