Change addShadowVolumeSceneNode to replace the current shadow completely. This allows to change the shadow parameters, such that a call to addShadowVolumeSceneNode will always result in the correct shadow. Note that redundant calls to this method will be quite expensive now, so avoid them.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@3477 dfc29bdd-3216-0410-991c-e03cc46cb475