DESCRIPTION:

Find the next voxel that carries a color mentionet in set,
starting at the current view position (which is clipped into
the volume, if required).
It seeks towards higher x/y/z coordinates if direction is
not given or 1, towards lower, if it is -1.
The current view position is wrapped to the middle of the found
voxel on return and its coordinates are given in $rc.
If no voxels are found, view position is unchanged and -1/-1/-1
is returned.