DESCRIPTION:

Fill the sphere/ellipsoid centered around x,y,z with toset.
Remarks only voxels that are the same color as x,y,z.
Doesn't touch greyscale voxels that are protected by an overlay.
If radiusy and/or radiusz are not given, they are copied form the
previous value.
If a given radius is 0, a circle is drawn in the appropriate
plane.