lack of a better name. This one behaves somewhat between Greedy and
    Always. It moves in if we can accelerate, unless the destination is
    clean and shouldn't be kept in framebuffer according to the score, in
    which case we migrate out (and force-migrate anything where migration
    is free). This should help fix lack of acceleration for drivers without
    UTS since removing exaAsyncPixmapGCOps, and has removed one performance
    trap with Radeon I'd noticed. It is the new default.