Standard callbacks: common routines and variables

Common routines and globals used by the standard callbacks.

vdsVec3 vdsViewPt: Position of the viewer
vdsVec3 vdsLookVec: Direction of the viewer's gaze
vdsFloat vdsFOV: Field of view, in radians
vdsFloat vdsInvTanFOV: We precompute 1/tan(FOV) for speed
vdsFloat vdsThreshold: Fraction of FOV which a node must cover to avoid being folded
vdsSetFOV: Tell VDSlib the current field of view, in radians
vdsSetViewpoint: Tell VDSlib the current viewpoint
vdsSetLookVec: Tell VDSlib the current "look vector"
vdsSetThreshold: Set the simplification threshold for the standard simplification callback

Common functions and global variables used by the standard routines provided by the view-dependent simplification package.