lua.wetgenes.grd.clip
gr=g:clip(x,y,z,w,h,d)
create a clipped window into this grd
the actual data is still stored in the original, so any changes there will effect the newly returned grd
x,y,z are the staring pixel and w,h,d are the width height and depth in pixels.
If you intend to use this clipped area for an extended period of time then you should duplicate this grd once you do this.
This returns a new grd with gr.parent set to g (the original grd)
This is a very shallow dangerous copy and should only really be used for temporary actions.