NAME                                                              (V36)
    InstallLayerHook -- safely install a new Layer->BackFill hook.

SYNOPSIS
    oldhook = InstallLayerHook( layer, hook )
    d0                          a0     a1

    struct Hook *InstallLayerHook( struct Layer *, struct Hook *);

FUNCTION
    Installs a new Layer->Backfill Hook, waiting until it is safe to do
    so. Locks the layer while substituting the new Hook and removing the
    old one. If a new Hook is not provided, will install the default layer
    BackFill Hook.

INPUTS
    layer - pointer to the layer in which to install the Backfill Hook.
    hook -  pointer to layer callback Hook which will be called
            with object == (struct RastPort *) result->RastPort
            and message == [ (Layer *) layer, (struct Rectangle) bounds,
                           (WORD) offsetx, (WORD) offsety ]

            This hook should fill the Rectangle in the RastPort
            with the BackFill pattern appropriate for offset x/y.

            If this hook pointer is NULL, the function installs
            the "default" Layers BackFill Hook into this Layer.

RESULTS
    oldhook - pointer to the Layer->BackFill Hook that was previously
              active.

BUGS

SEE ALSO
    graphics/clip.h utility/hooks.h