createSubset needs the x and y corners of the brush rectangle, the
index of the animation frame (to sync up the projections) and
a LimnViewer (the graphics element) to allow the subset data to
be added to the collection of renderable objects
will reseed the basis generator or reset if reading basis from a file
this allows the TourEngine to seek to a specific index in a set tour
(littletour or filebasistour), and allows a random tour to reset if
something has gone strange.