Define here the region of space to where the user 
is looking by default when creating a new layer.
Initially, the user is at (0, 0, +z), looking down to 
(0, 0, 0), and the up direction is (0, 1, 0), so the x axis 
points to the right, the y axis points to the top, and the z 
axis points to the user.
Eye
Set the user position, that is applied by default, when creating 
a new layer. Any position in 3D space is valid.
Center
Set the position to where the user is looking, that is applied by 
default, when creating a new layer. The distance from 
Eye
to 
Center must be within the limits of visibility (from
Near to 
Far).
Up
Set the direction pointing up, to the top of the screen,
that is applied by default, when creating a new layer. This
direction is converted internally to a unit vector normal to 
the direction 
Eye -> 
Center, so any non-zero
vector that is not parallel to that direction is valid.
The user position and orientation can be defined by a (X,Y,Z)
layer referential, pointing from the middle of the screen to: the
right (X), the top (Y), the user (Z). 
Eye is the origin,
Eye <- 
Center defines the Z axis, and 
Up
the Y axis, permiting to get the X axis by the cross product
of the other two axes. Initially, the axes are orientated along
(1,0,0) for X, (0,1,0) for Y and (0,0,1) for Z, but this is no
longer the case when the layer referential is rotated.
Each layer has its own position parameters, so in a window with
multiple visible layers, the image rendered in the graphic area 
is the superposition of the various, totally independent, layer 
views.