DESCRIPTION:

Mark a plane that goes through x y z (voxel coordinate system!)
which is perpendicular to px py pz (camera coordinate system!).
tolmin and tolmax can be used to specify, if e.g. a small seam
around the clicked voxel should be remarked (e.g. -0.9 0.9 for
tolmin/max will yield that result), or rather e.g. the voxels
on the side the normal points to (0 1e10) or the other side
(-1e10 0) should be remarked.
Note, that this uses a BFS style fill, so only connected voxels
will be remarked.
If newclass is not given, it is copied from the colorize variable,
if oldclasses are not given, it is copied from the voxel at x/y/z.