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.

1 files changed, 1 insertions, 1 deletions

diff --git a/src/gallium/auxiliary/util/u_screen.c b/src/gallium/auxiliary/util/u_screen.c
index 27f51e0898e..410f17421e6 100644
--- a/src/gallium/auxiliary/util/u_screen.c
+++ b/src/gallium/auxiliary/util/u_screen.c
@@ -356,7 +356,7 @@ u_pipe_screen_get_param_defaults(struct pipe_screen *pscreen,
       return 0;
 
    case PIPE_CAP_MAX_FRAMES_IN_FLIGHT:
-      return 2;
+      return 1;
 
    case PIPE_CAP_DMABUF:
 #ifdef PIPE_OS_LINUX