camera_bbox fails on very large DEMs

[ScottMcMichael]

If camera_bbox() is called with quick=false and the DEM is very large, say a constant DEM that covers half the planet, the function will return an enormous value for the bbox size. This is probably because points on the other side of the earth successfully project in to the camera. This can be seen if it is called in the ASP mapproject tool with quick=false, IceBridge data, and a datum DEM.