(int) SetGrid(interval, snap, win)
This function sets the grid parameters for the window indicated by the
third argument, which is 0 for the main window or 1-4 for the
subwindows. The interval argument sets the grid line spacing,
in microns. The snap number supplies the number of snap points
per grid interval if positive, or the number of grid intervals per
snap point if negative. Values from -10 - 10 are accepted for this
parameter. Values of 0.01 or larger are valid for the interval,
if not 0. Either argument can be 0, in which case the present
parameter is retained. The function fails if the window argument does
not correspond to an existing window.
The function does not redraw the window. The Redraw function
can be called to redraw the window if necessary.