Fix for vanishing-model problem: models are drawn in the same scene

Fix for vanishing-model problem: models are drawn in the same scene
graph as the terrain, except for internal cockpit view.  The SSG
scene-graph variables (except for the lighting root -- I'll get that
later) are now held in globals.hxx.

FGModelMgr::draw() is obsolete; I'll remove it in a future revision.