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.