Fix up the fast-path compositing operators; those are useful for
    sources without alpha, but can't be used for sources with
    alpha. Also, replaced fbCompositeSrcSrc_nxn with call to fbBlt as
    this function must handle 1, 4, 8, 16, 24, 32 bpp objects. Would
    be nice to optimize fbBlt for common cases involving 8, 16, 24 and
    32bpp.

    From Keith Packard.