[cogl/cogl-1.18] glx: Precisely wait for the next vblank

[Robert Bragg <rbragg src gnome org>]

commit 5ccd2825c724267d066c4e93190a4bec1ae0ced0
Author: Chris Wilson <chris chris-wilson co uk>
Date:   Fri Jan 30 00:16:16 2015 +0000

    glx: Precisely wait for the next vblank
    
    Currently the code queries the current msc then tries to approximate the
    value of the next msc satisfing the modulus 2 for when to wait. This
    introduces some instability as the msc may tick over during the
    roundtrip leading to a 32ms wait instead of a 16ms wait. This happens
    often enough to cause jerky animations, and affect gnome-shell-perf-tool.
    
    A simpler solution is just use a single roundtrip by using WaitForMsc to
    ask the driver to compute the next vblank itself.
    
    Cc: Owen W. Taylor <otaylor fishsoup net>
    Cc: Robert Bragg <robert linux intel com>
    Reviewed-by: Robert Bragg <robert sixbynine org>

 cogl/winsys/cogl-winsys-glx.c |    4 +---
 1 files changed, 1 insertions(+), 3 deletions(-)
---
diff --git a/cogl/winsys/cogl-winsys-glx.c b/cogl/winsys/cogl-winsys-glx.c
index 0487b79..34fb071 100644
--- a/cogl/winsys/cogl-winsys-glx.c
+++ b/cogl/winsys/cogl-winsys-glx.c
@@ -1626,10 +1626,8 @@ _cogl_winsys_wait_for_vblank (CoglOnscreen *onscreen)
           int64_t msc;
           int64_t sbc;
 
-          glx_renderer->glXGetSyncValues (xlib_renderer->xdpy, drawable,
-                                          &ust, &msc, &sbc);
           glx_renderer->glXWaitForMsc (xlib_renderer->xdpy, drawable,
-                                       0, 2, (msc + 1) % 2,
+                                       0, 1, 0,
                                        &ust, &msc, &sbc);
           info->presentation_time = ust_to_nanoseconds (ctx->display->renderer,
                                                         drawable,