present: Queue flips for later execution

When a flip (or unflip) is pending and a flip request comes in, leave
it queued until the pending flip completes and then execute it.

This fixes a bug where an application submitting back-to-back
present_pixmap requests for sequential frames would alternate between
flipping and copying as the pending flip would cause the new
present_pixmap request to not use a flip.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Tested-by: Frank Binns <frank.binns@imgtec.com>

present/present_priv.h

@@ -71,6 +71,7 @@ struct present_vblank {
     int                 num_notifies;
     Bool                queued;         /* on present_exec_queue */
     Bool                flip;           /* planning on using flip */
+    Bool                flip_ready;     /* wants to flip, but waiting for previous flip or unflip */
     Bool                sync_flip;      /* do flip synchronous to vblank */
     Bool                abort_flip;     /* aborting this flip */
 };