NAME
        WhichLayer -- Which layer is this point in?

    SYNOPSIS
        layer = WhichLayer( li, x, y )
        d0                  a0  d0 d1

        struct layer *whichlayer(struct layer_info*, word, word);

    FUNCTION
        Starting at the topmost layer check to see if this point (x,y)
            occurs in this layer.  If it does return the pointer to this
            layer. Return NULL if there is no layer at this point.

    INPUTS
        li  = pointer to LayerInfo structure
        (x,y) = coordinate in the bitmap

    RESULTS
        layer - pointer to the topmost layer that this point is in
                NULL if this point is not in a layer

    SEE ALSO
        graphics/layers.h