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