SYNOPSIS:
    MARKC_CUTSPHERE x y z toset radiusx [radiusy] [radiusz]

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.
