NAME
        BeginUpdate -- Prepare to repair damaged layer.

    SYNOPSIS
        result = BeginUpdate( l )
        d0                    a0

        long beginupdate( struct layer *);

    FUNCTION
        Convert damage list to cliprect list and swap in for
        programmer to redraw through. This routine simulates
        the ROM library environment. The idea is to only render in the
        "damaged" areas, saving time over redrawing all of the layer.
        The layer is locked against changes made by the layer library.

    INPUTS
        l - pointer to a layer

    RESULTS
        result - TRUE if damage list converted to cliprect list successfully.
                 FALSE if list conversion aborted. (probably out of memory)

    BUGS
        If BeginUpdate returns FALSE, programmer must abort the attempt to
        refresh this layer and instead call EndUpdate( l, FALSE ) to restore
        original cliprect and damage list.

    SEE ALSO
        endupdate(), graphics/layers.h, graphics/clip.h