st/dri: decrease input lag by syncing sooner in SwapBuffers
It's done by: - decrease the number of frames in flight by 1 - flush before throttling in SwapBuffers (instead of wait-then-flush, do flush-then-wait) The improvement is apparent with Unigine Heaven. Previously: draw frame 2 wait frame 0 flush frame 2 present frame 2 The input lag is 2 frames. Now: draw frame 2 flush frame 2 wait frame 1 present frame 2 The input lag is 1 frame. Flushing is done before waiting, because otherwise the device would be idle after waiting. Nine is affected because it also uses the pipe cap.
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