NAME
        ScrollLayer -- Scroll around in a superbitmap, translate coordinates
                      in non-superbitmap layer.

    SYNOPSIS
        ScrollLayer( dummy, l, dx, dy )
                     a0     a1 d0  d1

        void ScrollLayer( long, struct layer *, long, long);

    FUNCTION
        For a SuperBitMap Layer:
        Update the SuperBitMap from the layer display, then copy bits
        between layer and superbitmap to reposition layer over different
        portion of SuperBitMap.
        For nonSuperBitMap layers, all (x,y) pairs are adjusted by
        the scroll(x,y) value in the layer.  To cause (0,0) to actually
        be drawn at (3,10) use ScrollLayer(-3,-10). This can be useful
        along with installclipregion to simulate intuition gzzwindows
        without the overhead of an extra layer.

    INPUTS
        dummy - unused
        l - pointer to a layer
        dx - delta to add to current x scroll value
        dy - delta to add to current y scroll value

    BUGS
        May not handle (dx,dy) which attempts to move the layer outside the
        layer's SuperBitMap bounds.

    SEE ALSO
        graphics/layers.h