Add a new migration scheme, called "Smart" for 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.
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