Constructs a ScanlineGridIterator that iterates over the integer grid
[xorigin, xorigin+width-1] x [yorigin, yorigin+height-1] in left-right,
top-down order.
Sets the maximum streamline length (in output image units), the length
that FLIC will follow a particular streamline to compute successive
convolutions.
If this is true then we step to the requested point in steps of maxstep,
and if it is false we step to the requested point directly in the one step requested.