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	InstallLayerInfoHook - Install a backfill hook for non-layer      (V39)

	d0                           a0 a1

	struct Hook *InstallLayerInfoHook(struct Layer_Info *,struct Hook *);

	This function will install a backfill hook for the Layer_Info
	structure passed.  This backfill hook will be used to clear the
	background area where no layer exists.  The hook function is
	passed the RastPort and the bounds just like the layer backfill
	hook.  Note that this hook could be called for any layer.

	li - pointer to LayerInfo structure

	hook -  pointer to layer callback Hook which will be called
	        with object == (struct RastPort *) result->RastPort
	        and message == [ (ULONG) undefined, (struct Rectangle) bounds ]

	        This hook should fill the Rectangle in the RastPort
	        with the BackFill pattern appropriate for rectangle given.

	        If hook is LAYERS_BACKFILL, the default backfill is
	        used.  (Same as pre-2.0)

		If hook is LAYERS_NOBACKFILL, there will be no
	        backfill.  (NO-OP).

	oldhook - Returns the backfill hook that was in the Layer_Info.
	          Returns LAYERS_BACKFILL if the default was installed.
	          Returns LAYERS_NOBACKFILL if there was a NO-OP hook.
	          Returns -1 if there was some failure.

	See the example in InstallLayerHook.  Note that both the Layer
	pointer and the OffsetX/Y values are not available in the
	LayerInfo backfill hook.

	When the hook is first installed, it is *NOT* called.  It is up
	to the application to know if it is safe to fill in the area.
	Since the hook will be called when a layer is deleted, the easiest
	way to have layers call this hook is to create and delete a backdrop
	layer that is the size of the area.

	Also, note that currently the first long word of the hook message
	contains an undefined value.  This value may look like a layer pointer.
	It is *not* a layer pointer.

	The RastPort you are passed back is the same one passed to the
	function.  You should *not* use "layered" rendering functions
	on this RastPort.  Generally, you will wish to do BitMap operations
	such as BltBitMap().  The callback is a raw, low-level rendering
	call-back.  If you need to call a rendering operation with a
	RastPort, make sure you use a copy of the RastPort and NULL the
	Layer pointer.


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